UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K

(Mark One)

For the fiscal year ended December 31, 2004

For the transition period from                      to                     

Commission file number 1-10777

Ambac Financial Group, Inc.

(Exact name of Registrant as specified in its charter)

(212) 668-0340

(Registrant’s telephone number, including area code)

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark whether the Registrant: (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the Registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes x No ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of Registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K. x

Indicate by check mark whether the registrant is an accelerated filer (as defined in Exchange Act Rule 12b-2) Yes x No ¨

The aggregate market value of voting stock held by non-affiliates of the Registrant as of the close of business on June 30, 2004 was $7,963,272,372 (based upon the closing price of the Registrant’s shares of the New York Stock Exchange on that date, which was $73.44). For purposes of this information, the outstanding shares of Common Stock which were owned by all directors and executive officers of the Registrant were deemed to be shares of Common Stock held by affiliates.

As of March 7, 2005, 108,942,699 shares of Common Stock, par value $0.01 per share, (net of 2,256 treasury shares) were outstanding.

Documents Incorporated By Reference

Portions of the Registrant’s Annual Report to Stockholders for the year ended December 31, 2004 are incorporated by reference into Parts II and IV hereof. Portions of the Registrant’s Proxy Statement dated March 24, 2005 in connection with the Annual Meeting of Stockholders scheduled to be held on May 3, 2005 are incorporated by reference into Part III hereof.

TABLE OF CONTENTS

Part I

GENERAL

Ambac Financial Group, Inc., headquartered in New York City, is a holding company whose subsidiaries provide financial guarantee products and other financial services to clients in both the public and private sectors around the world. Ambac Financial Group was incorporated on April 29, 1991. Ambac Financial Group provides financial guarantees for public finance and structured finance obligations through its principal operating subsidiary, Ambac Assurance Corporation. Through its financial services subsidiaries, Ambac Financial Group provides financial and investment products including investment agreements, interest rate and total return swaps and funding conduits, principally to its clients of the financial guarantee business, which include municipalities and other public entities, health care organizations and asset-backed and structured finance issuers. Information about Ambac Financial Group is available through our website at http://www.ambac.com. In addition, our press releases and filings with the Securities and Exchange Commission (“SEC”) are available free of charge on the investor relations portion of our website.

Ambac Assurance and its subsidiary Ambac Assurance UK Limited, which serve the global capital markets, are primarily engaged in guaranteeing public finance and structured finance obligations. Ambac Assurance is the successor to the founding financial guarantee insurance company, which wrote the first bond insurance policy in 1971. Financial guarantee insurance policies written by Ambac Assurance generally guarantee payment when due of the principal of and interest on the guaranteed obligation. Ambac Assurance seeks to minimize the risk inherent in its financial guarantee portfolio by maintaining a diverse portfolio which spreads its risk across a number of criteria, including issue size, type of obligation, geographic area and obligor.

Ambac Assurance has earned triple-A ratings, the highest ratings available from Moody’s Investors Service, Inc. (“Moody’s”), Standard & Poor’s Ratings Services (“S&P”), Fitch, Inc. (“Fitch”) and Rating and Investment Information, Inc. (“R&I”). These ratings are an essential part of Ambac Assurance’s ability to provide credit enhancement. See “Rating Agencies” section.

Ambac Credit Products LLC, a wholly owned subsidiary of Ambac Assurance, primarily provides credit protection in the global markets in the form of structured credit derivatives. These structured credit derivatives, which are privately negotiated contracts, provide the counterparty with credit protection against the occurrence of a specific event such as a payment default or bankruptcy relating to an underlying obligation. Upon a credit event, Ambac Credit Products is required to either (i) purchase the underlying obligation at its par value and a loss is realized for the difference between the par and market value of the underlying obligation or (ii), make a payment equivalent to the difference between the par value and market value of the underlying obligation. Substantially all of Ambac’s structured credit derivative contracts relate to senior tranches of structured finance transactions are partially hedged with various financial institutions or structured with first loss protection. Structured credit derivatives issued by Ambac Credit Products are insured by Ambac Assurance. See “Management’s Discussion and Analysis — Risk Management” in Ambac Financial Group’s 2004 Annual Report to Stockholders for more detail about structured credit derivatives.

In addition to the guarantees on fixed income obligations described above, Ambac, from time to time, enters into transactions that expose the company to risks which may not be

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correlated to credit risk, for example weather-related or other disasters, mortality or other property and casualty type risk characteristics. Ambac underwrites such risks so that first loss must occur before Ambac would become liable in respect of such risks. Additionally, Ambac underwrites such business primarily in relation to broad indices and reference pools which embody diverse risk characteristics.

Ambac Financial Group’s investment agreement business, conducted through its subsidiary, Ambac Capital Funding, Inc., provides investment agreements primarily to municipalities and other public entities, issuers of structured finance obligations and international issuers. Investment agreements issued by Ambac Capital Funding are insured by Ambac Assurance. Investment agreements are primarily used by issuers to invest bond proceeds until the proceeds can be used for their intended purpose. The investment agreement provides for the guaranteed return of principal invested, and for the payment of interest thereon at a guaranteed rate. See “Investment Agreements” section.

Ambac Financial Group provides interest rate and currency swaps through its subsidiary Ambac Financial Services, LLC, primarily to states, municipalities and their authorities, issuers of asset-backed securities and other entities in connection with their financings. Ambac Financial Group also enters into total return swaps with professional counterparties through its subsidiary Ambac Capital Services LLC. Total return swaps are generally used for fixed income obligations, which meet Ambac Assurance’s credit underwriting criteria. See “Derivative Products” section.

As a holding company, Ambac Financial Group, Inc. is largely dependent on dividends from Ambac Assurance to pay dividends on its capital stock, to pay principal and interest on its indebtedness, to pay its operating expenses, to purchase its common stock in the open market and to make capital investments in its subsidiaries. Dividends from Ambac Assurance are subject to certain insurance regulatory restrictions. See “Insurance Regulatory Matters — Wisconsin Dividend Restrictions” section and “Management’s Discussion and Analysis — Liquidity and Capital Resources” in Ambac Financial Group’s 2004 Annual Report to Stockholders.

FORWARD-LOOKING STATEMENTS

Materials in this annual report may contain information that includes or is based upon forward-looking statements within the meaning of the Securities Litigation Reform Act of 1995. Forward-looking statements are not historical facts but instead represent Ambac’s belief regarding future events, many of which, by their nature, are inherently uncertain and outside our control. You can identify these statements by the fact that they do not relate strictly to historical or current facts and relate to future plan or objectives and results.

Any or all of Ambac’s forward-looking statements here or in other publications may turn out to be wrong and are based on current expectations and the current economic environment. Ambac’s actual results may vary materially, and there are no guarantees about the performance of Ambac’s securities. Among factors that could cause actual results to differ materially are: (1) changes in the economic, credit or interest rate environment in the United States and abroad; (2) the level of activity within the national and worldwide debt markets; (3) competitive conditions and pricing levels; (4) legislative and regulatory developments; (5) changes in tax laws; (6) the policies and actions of the United States and other governments and (7) other risks and uncertainties that have not been identified at this time. Ambac is not obligated to publicly correct or update any forward-looking statement if we later become aware that it is not likely to be achieved, except as required by law. You are advised, however, to

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consult any further disclosures we make on related subjects in Ambac’s reports to the Securities and Exchange Commission.

BUSINESS SEGMENTS

The following paragraphs describe the business operations of Ambac Financial Group, Inc. and its subsidiaries for its two reportable segments: Financial Guarantee and Financial Services.

Financial Guarantee

Generally, financial guarantee insurance written by Ambac Assurance guarantees to the holder of the underlying obligation timely payment of principal and interest on such obligation in accordance with its original payment schedule.

Financial guarantee insurance is a form of credit enhancement that benefits both the issuer and the investor. Issuers benefit because their insured securities are sold with the highest available credit rating, resulting in interest cost savings and greater marketability. In addition, for complex financings and obligations of issuers that are not well known by investors, credit enhanced obligations receive greater market acceptance than obligations without credit enhancement. Investors benefit from greater marketability, secondary market price stability, active credit surveillance and protection from loss associated with issuer default.

Ambac Financial Group derives financial guarantee revenues from: (i) premiums earned from insurance contracts; (ii) net investment income; (iii) revenue from credit derivative transactions; (iv) net realized gains and losses from sales of investment securities; and (v) certain structuring and other fees. Financial guarantee revenues were $1,168.7 million, $1,033.6 million, and $816.2 million in 2004, 2003, and 2002, respectively. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and Note 18 of Notes to Consolidated Financial Statements in Ambac Financial Group’s 2004 Annual Report to Stockholders.

Financial guarantee products are sold in three principal markets: the U.S. public finance market, the U.S. structured finance and asset-backed market, and the international finance market. Total gross par guaranteed for the years ended December 31, 2004, 2003 and 2002 were $118.1 billion, $115.3 billion, and $116.4 billion, respectively.

U. S. Public Finance Market

The U.S. public finance market includes taxable and tax-exempt bonds, notes and other evidences of indebtedness issued primarily by states, political subdivisions (e.g., cities, counties and towns), water, sewer, electric and other utility districts, airports, higher educational institutions, hospitals, transportation and housing authorities. Public finance obligations are generally supported by either the taxing authority of the issuer or the issuer’s or underlying obligor’s ability to collect fees or assessments for certain projects or public services. More recently, the public finance market has expanded to include structured, project finance and asset-backed bond issues for infrastructure projects, sports stadiums, and other municipal purposes. This portion of the market is growing and has become a focus for Ambac Assurance in recent years. The following table sets forth the volume of new issues of long-term (longer

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than 12 months) public finance bonds and the volume of new issues of insured long-term public finance bonds over the past ten years in the United States.

U.S. Public Finance Long-Term Market

The foregoing table illustrates the changes in the total volume and insured volume of new issues of public finance bonds over the past ten years. Changes in volume of public finance bond issuance during this period are primarily attributable to changes in refunding activity related to the then-current interest rate environment, along with the issuers new money requirements. Total volume in 2002 and 2003 were record issuances as a result of the low interest rate environment and considerable infrastructure finance needs. Total volume in 2004 was down 6% from the record set the previous year. Insured volume, as a percentage of total volume (“insured penetration”), which had grown consistently from 1995 through 1998, declined during 1999 and 2000. The decline during 2000 is generally considered to have resulted from the combination of the relatively high credit quality of issues that came to market during the period and the firmness in premium pricing in the industry. During 2001 through 2004, the insured penetration has increased, largely the result of budget deficits experienced by municipalities and the corresponding flight to quality by investors.

Ambac Assurance guaranteed gross par of $44.6 billion, $43.0 billion and $42.3 billion in 2004, 2003 and 2002, respectively, in the U.S. public finance market.

In the U.S. public finance market, an issuer typically pays an up-front premium to Ambac Assurance at the time the policy is issued. Premiums are usually quoted as a percentage of the total amount of principal and interest that is scheduled to become due during the life of the insured bonds.

Proposed new public finance bond issues are submitted to Ambac Assurance by issuers (or their investment bankers or financial advisors) to determine their suitability for financial insurance. Public finance bond issues are sold on either a competitive or a negotiated basis. With respect to competitive issues, an issuer will publish a notice of sale soliciting bids for the purchase of a proposed issue of bonds. Potential bidders on the bonds then form syndicates. These syndicates then solicit a determination from some or all of the financial guarantors whether an issue is suitable for financial guarantee and at what premium rate and on what terms. The syndicate then determines whether to bid on the issue with a financial guarantee (and if so, with which financial guarantor) or without a financial guarantee. The issuer then generally selects the syndicate with the lowest bid. In a negotiated offering, the issuer has

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already selected an investment bank and that investment bank solicits premium quotes and terms from the financial guarantors.

Ambac Assurance also provides financial guarantees on public finance bonds outstanding in the secondary market that are typically purchased by an institution to facilitate the sale of bonds in its portfolio or inventory. The financial guarantee generally increases the sale price of bonds (typically by an amount greater than the cost of the policy) and affords a wider secondary market and therefore greater marketability to a given issue of previously issued bonds. As is the case with new issues, the premium is generally payable in full at the time of policy issuance. Ambac Assurance employs the same underwriting standards on secondary market issues that it does on new public finance issues.

As of December 31, 2004 and 2003, net outstanding par exposure related to public finance bond transactions was $239.7 billion and $215.3 billion, respectively. See “Financial Guarantees in Force—Types of Bonds” section, for a breakout of net outstanding par exposure by bond type.

U.S. Structured Finance and Asset-backed Market

Financial guarantees of securities in the U.S. structured finance and asset-backed market are typically issued in connection with transactions in which the securities being issued are secured by or payable from a specific pool of financial or tangible assets. This pool of assets has an identifiable cash flow or market value and is generally held by a special purpose entity. Structured finance and asset-backed obligations insured by Ambac Assurance generally have the benefit of over-collateralization and/or other forms of credit enhancement to mitigate credit risks associated with the related assets. These forms of credit enhancement are designed to absorb the expected losses in these transactions.

Structured finance obligations include the securitization of a variety of asset types such as mortgages, home equity loans, auto loans, student loans, credit card debt, leases, commercial asset-backed securities and pooled debt obligations originated in the United States (“Structured Finance”). Currently, the largest component of Ambac’s Structured Finance business relates to the securitization of mortgages and home equity loans. Another target area in Structured Finance is the credit enhancement of pooled debt obligations, including structured credit derivatives. These transactions involve the securitization of a diverse portfolio of corporate bonds and loan obligations and asset-backed securities (the “Securitized Assets”). The transaction structure provides certain financial protection to Ambac. This financial protection can take several forms, however, the most common are over-collateralization, first loss retention and excess spread. In the case of over-collateralization (i.e., the principal amount of the Securitized Assets exceeds the principal amount of the structured finance obligations guaranteed by Ambac Assurance), the structure allows the transaction to experience defaults among the Securitized Assets before a default is experienced on the structured finance obligations that have been guaranteed by Ambac. In the case of first loss retention, the financial guarantee insurance policy only covers a senior layer of losses. A subordinated layer of losses is either retained by the seller or sold off in the form of equity and mezzanine debt to other investors. In the case of excess spread, the financial assets generate cash flow in the form of interest that is in excess of the interest payments on the related debt. All or a portion of this excess spread is applied to redeem structured finance obligations, thus creating over-collateralization.

Structured finance also encompasses credit enhancement for asset-backed commercial paper conduits (“conduits”). Conduits are used by issuers to efficiently fund assets in the

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short-term commercial paper market. Typically sponsored by financial institutions, the conduits usually purchase financial assets and asset-backed securities, and issue commercial paper to fund the purchase of the assets. The typical conduit structure provides Ambac with significant credit protection prior to a claim on Ambac’s insurance policy. A conduit requires program-wide credit enhancement as one of several elements needed to support the conduit’s credit rating for the structure, of which Ambac provides a senior portion.

Unlike the public finance market in which a substantial portion of the deals is bid competitively by the financial guarantors, the structured and asset-backed market is essentially a negotiated one. The financial guarantor will work directly with the investment bank or client to create an acceptable structure once having been awarded the business.

The U.S. structured finance and asset-backed market in which Ambac Assurance provides financial guarantees is broad and varied, comprising public issues and private placements. The increasing array of classes of assets securitized or guaranteed, and the recent rapid changes to the market, makes estimating the size of the markets that we participate in difficult.

Premiums for structured finance and asset-backed policies are typically based on a percentage of principal insured. The timing of the collection of structured finance and asset-backed premiums can be collected in a single payment at policy inception date or collected periodically (e.g., monthly, quarterly or annually) from the cash flow generated by the underlying assets.

As of December 31, 2004 and 2003, net outstanding par exposure related to U.S. structured finance and asset-backed transactions was $132.5 billion and $124.1 billion, respectively. See “Financial Guarantees in Force—Types of Bonds” section, for a breakout of net outstanding par exposure by bond type.

International Finance Market

Outside of the United States, structured and asset-backed issuers, utilities, sovereign and sub-sovereign issuers, and other issuers are increasingly using financial guarantee products, particularly in markets throughout Western Europe. A number of important trends in international finance markets have contributed to this expansion. In the United Kingdom, ongoing privatization efforts have shifted certain risks associated with the development or operation of infrastructure projects from the government to market participants, thus prompting investors in such projects to seek the security of financial guarantee products. These privatization efforts are currently being initiated in other European countries, including Spain, Portugal and Italy. In Western Europe and the emerging markets, there also is growing interest in asset-backed securitization.

While the principles of securitization have been increasingly applied in overseas markets, development in particular countries has varied as a result of the relative sophistication of the local capital markets and the impact of legal and financial regulatory requirements. It is anticipated that securitization will continue to expand internationally, albeit at varying rates in each country. Ambac Assurance, and through its United Kingdom subsidiary Ambac Assurance UK Limited, insures a wide array of obligations in the international finance markets including infrastructure finance, asset-backed and structured finance transactions, utilities, whole company securitizations (i.e. securitizations of substantially all of the operating assets of corporate credits typically engaged in the provision of utility services or infrastructure) and other obligations in selected international finance markets.

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Ambac Assurance’s strategy in the international finance markets is to strengthen its franchise in developed markets by focusing on high quality infrastructure, structured finance, securitization, and utility finance transactions, and in emerging markets by focusing on top tier future flow transactions (structured transactions secured by U.S. Dollar cash flows generated from exports or payment remittances) and pooled debt obligations.

Ambac UK, which is authorized and regulated in the United Kingdom to provide certain classes of general financial guarantees (and is also authorized to conduct business throughout much of the European Union), has been Ambac Assurance’s primary vehicle for directly issuing financial guarantee policies in the United Kingdom and European Union. In February 2005, Ambac UK has established a branch office in Milan, Italy. Ambac UK has entered into net worth maintenance and reinsurance agreements with Ambac Assurance, which support its triple-A ratings.

Ambac Assurance is party to an alliance in Japan with Sompo Japan Insurance Inc. (“Sompo Japan”). Although the development of the Japanese securitization market has been slow, we believe that this alliance is competitively positioned for future growth.

While there is evidence that the volume of international structured finance transactions has increased significantly in the recent past, unlike the public finance and domestic asset-backed markets, there are few statistics that effectively track volume in the global markets. There are several reasons for this, including the varied nature of the deals coming to market, the early stages of development of certain asset classes and the fact that many international deals are privately placed.

Premiums for international finance policies are based on a percentage of either principal or principal and interest insured. The timing of the collection of international finance premiums varies among individual transactions; some being collected in a single payment at policy inception date, and others being collected periodically (i.e., monthly, quarterly or annually).

As of December 31, 2004 and 2003, net outstanding par exposure related to international finance transactions was $87.3 billion and $86.4 billion, respectively. See “Financial Guarantees in Force - Types of Bonds” section, for a breakout of net outstanding par exposure by bond type.

Risk Management

Ambac Financial Group has a Portfolio Risk Management Committee (“PRMC”) which has established various procedures and controls to monitor and manage credit risk. The PRMC consists of senior risk management professionals and senior management of Ambac Financial Group. Its purview is enterprise-wide and its focus is on risk limits and measurement, concentration and correlation of risk, and the attribution of economic and regulatory capital in a portfolio context.

Underwriting guidelines, policies and procedures have been developed by Ambac Assurance’s management with the intent that Ambac Assurance guarantees only those obligations which, in the opinion of Ambac Assurance underwriting officers, are of investment grade quality with a remote risk of loss. However, losses may occur from time to time and it is Ambac Assurance’s policy to provide for loss reserves on non-derivative insurance policies that are adequate to cover probable and estimable losses. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and Note 2 and Note 6 of Notes to

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Consolidated Financial Statements in Ambac Financial Group’s 2004 Annual Report to Stockholders.

The underwriting process involves review of structural, legal, political and credit issues, including compliance with current Ambac Assurance underwriting standards. These standards are reviewed periodically by management. Additionally, the underwriting process often entails on-site due diligence covering the parties to the transaction, such as the issuer, originator, servicer or manager.

The decision to guarantee an issue is based upon such credit factors as the issuer’s ability to repay the bonds, the bond’s security features and the bond’s structure, rather than upon an actuarial prediction of the likelihood that the issuer will default on the underlying debt obligation.

Members of Ambac Assurance’s underwriting staff review all requests for guarantees. The underwriting process is designed to screen issues and begins with a credit analysis by the primary analyst assigned to the issue. The credit is then reviewed within the primary analyst’s underwriting group. At a minimum, the primary analyst’s recommendation to qualify or reject an issue must be approved by a concurring analyst and a credit officer. The number of additional approvals required for a particular credit depends in part on Ambac Assurance’s aggregate exposure to the credit. In some cases, the complexity of the credit or whether it is a new asset type are determining factors in the approval/review process. Ambac Assurance has three established credit committees comprised of senior credit officers, credit and market risk managers and attorneys. All Structured and International Finance credits and large, complex or new types of Public Finance credits, recommended by the underwriting group, must be formally presented to the credit committee for approval. For certain Public Finance issues, the primary analyst’s recommendation must be approved by a concurring analyst and credit officer and need not be formally presented to the credit committee for approval.

Ambac Assurance assigns internal ratings to individual exposures as part of the underwriting process and at surveillance reviews. These internal ratings, which represent Ambac Assurance’s independent judgments, are based upon underlying credit parameters similar to those used by rating agencies.

Public Finance Underwriting:

In addition to general underwriting standards, each asset class, and bond type within each asset class, has more specific underwriting criteria. For example, the critical risk factors for public finance credits will include the credit quality of the issuer, type of issue, the repayment source, the type of security pledged, the presence of restrictive covenants, and the bond’s maturity. Each bond issue is evaluated in accordance with, and the final premium rate is a function of, the particular factors as they relate to such issue.

Underwriting criteria that are applied for each bond type reflect the differences in, for example, economic and social factors, debt management, project essentiality, financial management, legal and administrative factors, revenue sources and security features.

Structured Finance Underwriting:

Structured finance and asset-backed obligations generally entail three forms of risks: asset risk, which relates to the amount and quality of the underlying assets; structural risk, which relates to the extent to which the transaction’s legal structure provides protection from loss; and execution risk, which is the risk that poor performance at the servicer or manager level contributes to a decline in cash flow available to the transaction. Ambac Assurance addresses these risks through its credit underwriting guidelines, standards and procedures.

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In general, the amount and quality of asset coverage required is determined by the historical performance of the underlying asset type or the transaction’s specific underlying assets. The future performance or value of the underlying pool of assets will generally determine whether the amount of over-collateralization or other credit enhancement ultimately is sufficient to protect investors, and therefore the guarantor, against adverse asset performance. The ability of the servicer or manager to properly service and/or manage the underlying assets often is a factor in determining future asset performance.

Structured and asset-backed securities are usually designed to protect the investors, and therefore the guarantor, from the bankruptcy or insolvency of the entity that originated the underlying assets as well as from the bankruptcy or insolvency of the servicer of those assets. The servicer of the assets is typically responsible for collecting cash payments on the underlying assets and forwarding such payments, net of servicing fees, to the special purpose entity. Other issues include whether the sale of the assets by the originator to the issuer would be respected in the event of the bankruptcy or insolvency of the originator and whether the servicer of the assets may be permitted or required to delay the remittance to investors of any cash collections held by it or received by it after the time it becomes subject to bankruptcy or insolvency proceedings.

Within the mortgage-backed and home equity loan market, Ambac Assurance seeks to work with higher quality, well-capitalized issuers. The issuers typically originate or purchase residential mortgages, home equity loans or home equity lines of credit, which are in turn sold by the issuers in the form of asset-backed securities. In considering whether to guarantee these securities, Ambac Assurance analyzes the quality of the underlying assets, the structure of the securitization, the experience and financial strength of the servicer of the underlying assets and the credit quality of the issuer.

The following table presents the top five servicers by net par outstanding, for the consumer asset-backed exposures, are as follows:

International Finance Underwriting:

In the international markets, Ambac Assurance seeks to guarantee transactions of the same high credit standards it applies in its U.S. business. However, an understanding of the unique risks related to the particular country and region that could impact the credit of the issuer is necessary. These risks include legal and political environments, capital market dynamics, foreign exchange issues, and the degree of governmental support. Ambac Assurance monitors these risks carefully and addresses them through its credit underwriting guidelines, (which include country limits), underwriting procedures and transaction documentation.

Geographically, the markets receiving Ambac Assurance’s primary international focus have been the United Kingdom, Australia, Italy, Germany and certain parts of Latin America. In addition, Ambac Assurance has guaranteed transactions in which the geographic risk is spread over multiple countries. The types of international obligations guaranteed have primarily been pooled debt obligations, asset-backed securities, special revenue and infrastructure obligations.

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Management believes that risk associated with its international book of business is similar in risk type to its domestic structured finance book of business and, in fact, international transactions may include components of domestic exposure.

Pricing:

Ambac Assurance determines premium rates on the basis of the type of transaction and its assessment of the risk it is assuming. Factors considered in pricing include term to maturity, structure of the issue, and credit and market factors including security features and other credit enhancement features. Additionally, the interest rate spread between insured and uninsured obligations with characteristics similar to those of the proposed bond issue is considered in the pricing process as well as the cost and the projected return to Ambac Assurance. The premium rate for a new issue also takes into account the benefits to be obtained by the issuer.

Overall, the business environment has become more competitive. This increased competition has had a moderately adverse impact on pricing. Credit spreads and pricing continue to provide adequate returns in most markets.

Surveillance and Remediation:

The Surveillance Group is responsible for monitoring outstanding financial guarantee exposures. Active surveillance enables Ambac’s Surveillance Group to track single credit migration or industry credit trends. Surveillance analysts review, on a regular and ad hoc basis, credits in the book of business. Risk-adjusted surveillance strategies have been developed for each transaction type. Review periods and scope of review vary by bond type based upon the risk inherent in the nature of the credits. The focus of the surveillance review is to determine credit trends and recommend appropriate classifications, ratings and review periods. Surveillance analysts and other credit professionals review the financial guarantee portfolio for concentration of risk by (i) specific bond types; (ii) geographic location; and (iii) size of issue. The separate underwriting groups are also responsible for portfolio analysis which entails a broader examination of trends in specific asset classes and bond types.

Surveillance of the credit quality of underlying reference obligations in the structured credit derivatives portfolio is performed on a regular basis. Credit spreads, which act as a measure of the market’s perception of an issuer’s credit quality, are monitored to identify potential problems. In addition, published credit ratings and current news reports are monitored regularly.

Those issues that are either in default or have developed problems that eventually may lead to a claim or loss are tracked closely by the appropriate surveillance team and reporting to management and Ambac’s Board of Directors by preparation of an adversely classified credit listing. Relevant information, along with the schedule of corrective actions, is reviewed in the regular remedial credit meetings. Internal and/or external counsel generally reviews the documents underlying any problem credit and if applicable, an analysis is prepared outlining Ambac Assurance’s rights and potential remedies, the duties of all parties involved and recommendations for corrective actions. Ambac Assurance also meets with relevant parties to the transaction as necessary. In many instances, Ambac Assurance, under the terms of the documents governing the underlying obligation, has the ability, among other things, to direct that audits be performed with respect to servicer and trustee contractual responsibilities.

The rating agencies also review the credits underlying Ambac Assurance’s financial guarantees and, in most cases, advise Ambac Assurance of the credit rating each transaction would receive if it were not insured.

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Financial Guarantees in Force

Ambac Assurance underwrites and prices financial guarantees on the assumption that the guarantee will remain in force until maturity of the underlying bonds. Ambac Assurance estimates that the average life of its guarantees on par in force at December 31, 2004 was 11 years. The 11 year average life is determined by applying a weighted average calculation, using the remaining years to expected maturity of each guaranteed bond, and weighting them on the basis of the remaining par guaranteed. No assumptions are made for future refundings of guaranteed issues.

Ambac Assurance seeks to maintain a diversified financial guarantee portfolio designed to spread its risk based on a variety of criteria, including issue size, type of bond, geographic area and issuer.

As of December 31, 2004, the total net par amount of guaranteed bonds outstanding was $459.4 billion.

Types of Bonds

The table below shows the distribution by bond type of Ambac Assurance’s guaranteed portfolio as of December 31, 2004.

Guaranteed Portfolio by Bond Type

as of December 31, 2004 ⁽¹⁾

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The table below shows the percentage, by bond type, of new business guaranteed by Ambac Assurance during each of the last three years.

New Business Guaranteed by Bond Type ⁽¹⁾