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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-K

For The Fiscal Year Ended December 31, 2003

OR

Commission File Number 1-3610

ALCOA INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone numbers:

Investor Relations (212) 836-2674

Office of the Secretary (412) 553-4707

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, and (2) has been subject to such filing requirements for the past 90 days. Yes þ 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. þ

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

The aggregate market value of the outstanding common stock, other than shares held by persons who may be deemed affiliates of the registrant, as of the last business day of the registrant’s most recently completed second fiscal quarter was approximately $22 billion. As of February 16, 2004, there were 870,294,908 shares of common stock, par value $1.00 per share, of the registrant outstanding.

Documents incorporated by reference.

Parts I, II and IV of this Form 10-K incorporate by reference certain information from the registrant’s 2003 Annual Report to Shareholders (Annual Report). Part III of this Form 10-K incorporates by reference certain information from the registrant’s definitive Proxy Statement for its 2004 Annual Meeting of Shareholders to be filed pursuant to Regulation 14A (Proxy Statement).

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Note on Incorporation by Reference

In this Form 10-K, selected items of information and data are incorporated by reference to portions of the Annual Report. Any reference in this report to disclosures in the Annual Report shall constitute incorporation by reference of that specific disclosure into this Form 10-K.

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ALCOA INC.

Formed in 1888 under the laws of the Commonwealth of Pennsylvania, Alcoa Inc. has its principal office in Pittsburgh, Pennsylvania. In this report, unless the context otherwise requires, Alcoa or the “company” means Alcoa Inc. and all subsidiaries consolidated for the purposes of its financial statements.

The company’s Internet address is http://www.alcoa.com. Alcoa makes available free of charge on or through its website its annual report on Form 10-K, quarterly reports on Form 10-Q, current reports on Form 8-K, and amendments to those reports filed or furnished pursuant to Section 13(a) or 15(d) of the Securities Exchange Act of 1934 as soon as reasonably practicable after the company electronically files such material with, or furnishes it to, the Securities and Exchange Commission.

The company makes available free of charge on its website its Business Conduct Policies and its Guide to Business Conduct which apply equally to employees at all levels of the company, including its senior officers and directors. Alcoa’s corporate governance guidelines and the charters of the audit, compensation and nominating committees of the company’s Board of Directors are also available free of charge on its website. This information is downloadable from the website or available in print to any shareholder who requests it by contacting Corporate Communications at 412-553-4545.

PART I

Item 1. Business.

Description of the Business

Information describing Alcoa’s businesses can be found in the Annual Report at the indicated pages:

Item

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Structure of Certain Operations

The company’s Alumina and Chemicals segment primarily consists of a series of affiliated operating entities referred to as Alcoa World Alumina and Chemicals (AWAC). Generally, Alcoa owns 60% and Alumina Limited (formerly WMC Limited) owns 40% of these entities. For more information on AWAC, see Exhibit Nos.10(a) through 10(e) to this report.

Alcoa Aluminio S.A. (Aluminio), an integrated aluminum producer in Brazil, is a subsidiary of Alcoa. Aluminio operates mining, refining, smelting and fabricated products facilities at various locations in Brazil. In 2003, Alcoa acquired the 40.9% shareholding in Aluminio held by affiliates of Camargo Correa S.A. (collectively the “Camargo Group”). Prior to the acquisition, Alcoa had owned approximately 59% of Aluminio and the Camargo Group had been the principal minority shareholder since 1984. For more information on this acquisition, see Note F to the financial statements.

Bauxite Interests

Aluminum is one of the most plentiful elements in the earth’s crust. Aluminum is produced primarily from bauxite, an ore containing aluminum in the form of aluminum oxide, commonly referred to as alumina. Aluminum is made by extracting alumina from bauxite and then removing oxygen from the alumina. Alcoa processes most of the bauxite that it mines into alumina. The company obtains bauxite from reserves held by AWAC, from the company’s interests in Brazil, and under both long-term and short-term contracts and mining leases. In 2003, Alcoa consumed 30.8 million metric tons (mt) of bauxite from its own reserves, 6.9 million mt from related third parties and 1.9 million mt from unrelated third parties. Alcoa’s present sources of bauxite are sufficient to meet the forecasted requirements of its alumina refining operations for the foreseeable future. The following table provides information regarding the company’s bauxite interests:

Alcoa Active¹ Bauxite Interests

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Alumina Refining Facilities and Capacity

Alcoa is the world’s leading producer of alumina. Alcoa’s alumina refining facilities and its worldwide alumina capacity are shown in the following table:

Alumina Refining Capacity

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In January 2003, Alcoa and BHP Billiton signed a Memorandum of Understanding (MOU) with the government of Suriname providing for (i) continuation and expansion of mining and refining activities in eastern Suriname and (ii) various exploration and other activities over the next two years relating to the feasibility of bauxite and alumina investments in western Suriname. Under the MOU, Alcoa and BHP Billiton have exclusive rights in western Suriname and have committed to spend up to $8.5 million over 21-25 months to explore this opportunity, shared 55% (Alcoa) and 45% (BHP Billiton). The MOU provides that Alcoa and BHP Billiton will negotiate an investment agreement with the Suriname government within 18 months.

In 2003, Alcoa broke ground on its previously announced capacity expansion at its Paranam alumina refinery in Suriname by 250,000 mt per year, an increase of approximately 13% that will bring its total capacity to approximately 2.2 million mt per year. The expansion is expected to be completed by July 2005. Alcoa and BHP Billiton have also signed an agreement extending the terms of their existing joint venture agreement on bauxite mining and alumina refining in eastern Suriname to 2025.

Primary Aluminum Facilities and Capacity

The company’s primary aluminum smelters and their respective capacities are shown in the following table:

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Alcoa Worldwide Smelting Capacity

Alcoa currently has approximately 562,000 mtpy of idled smelting capacity out of a worldwide, consolidated primary aluminum capacity of 4,020,000 mtpy.

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Alcoa owns interests in the following primary aluminum facilities that are accounted for on the equity or cost basis method. The capacity associated with these facilities is not included in Alcoa’s consolidated capacity.

In February 2004, Alcoa announced that it had sold its 10 percent stake in the Alscon smelter in Ikot Abasi, Nigeria to the Federal Government of Nigeria. The registration of the change in control of the shares is still pending. The Alscon smelter was 70 percent owned by the Federal Government and 20 percent held by Ferrostaal AG. Only a portion of the facility has ever been operated and has been idle since mid-1999. The Bureau of Public Enterprise, an agency of the Nigerian government, is working to privatize Alscon. In connection with the sale, the company also terminated its entitlement to purchase all aluminum produced at the smelter for export.

In 2003, Alcoa finalized agreements with the Government of Iceland and Landsvirkjun, Iceland’s National Power Company, to build the 322,000 mt per year, Fjardaál aluminum facility in eastern Iceland. Alcoa chose Bechtel Group, Inc. and its partner, HRV, an Icelandic engineering consortium, to design and build this new plant. It is scheduled to begin production in 2007, with construction of the plant to begin in 2005.

Alcoa also will explore the aluminum smelting and associated hydroelectric power opportunities in western Suriname. In January 2003, Alcoa signed an MOU with the government of Suriname providing for an 18-month exclusive period of investigation of the feasibility of smelting and associated hydroelectric power investment in western Suriname. Under the MOU Alcoa has budgeted $1.5 million - $7.5 million in expenditures over this period to assess the investment potential and to negotiate an investment agreement with the government.

The MOU with the Quebec government under which Alcoa planned to invest C$1 billion over eight years for the expansion and upgrade of its Baie Comeau, Quebec aluminum smelter expired by its terms on December 31, 2003 for failure of the parties to execute definitive agreements. The parties have agreed to extend the deadline until February 29, 2004.

In September 2003, Alcoa signed an MOU with the government of the Kingdom of Bahrain permitting Alcoa to acquire up to a 26% equity stake in Alba, a Bahrain company that owns and operates a 512,000 mtpy aluminum smelter. The MOU also addresses a long-term alumina supply arrangement for the Alba smelter.

Energy

Alcoa produces aluminum from alumina by an electrolytic process requiring large amounts of electric power. Electric power accounts for approximately 25% of the company’s primary aluminum costs. Alcoa generates

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approximately 25% of the power used at its smelters worldwide and generally purchases the remainder under long-term arrangements. The paragraphs below summarize the sources of power and material long-term power arrangements for Alcoa’s smelters.

North America - Electricity

For its 13 North American smelters, the company (largely through its wholly-owned subsidiary, Alcoa Power Generating Inc. (APGI)) generates approximately 25% of the power requirements, and generally purchases the remainder under long-term contracts. APGI owns and operates two hydroelectric projects consisting of eight dams under Federal Energy Regulatory Commission licenses, which are up for renewal in 2005 and 2008.

In the Pacific Northwest, Alcoa obtains approximately half of its power needs for its Wenatchee smelter under a contract through 2011 with Chelan County Public Utility District located in the State of Washington. In addition, Alcoa has a contract through 2006 with the Bonneville Power Administration (BPA) that serves part of the Wenatchee smelter, as well as the Intalco smelter. Alcoa is currently returning part of its allotment of power to BPA under arrangements that end in September 2006.

The company, through APGI, generates substantially all of the power used at its Warrick smelter using nearby coal reserves. A 1996 coal supply contract satisfies up to 70% of the smelter’s fuel requirements through 2006. Annual contracts satisfy the remainder of the fuel requirements. In April 2001, under the terms of an operating agreement, the company assumed operation of the power plants that supply the Warrick smelter from Southern Indiana Gas & Electric Company until at least 2008.

The Rockdale smelter uses lignite supplied by the company’s Sandow Mine to generate power. The company has received all necessary permits to open a new lignite mine, the Three Oaks Mine, on land it owns or controls adjacent to its existing Sandow Mine. Company-owned generating units supply about one-half of the total electricity requirements of the smelter. TXU Energy supplies the balance through a long-term power contract that does not expire until at least 2038, with the parties having the right to terminate the contract after 2013 if there has been an unfavorable change in law or after 2025 if the cost of the electricity exceeds the market price.

APGI hydroelectric facilities provide electric power for the aluminum smelters at Alcoa, Tennessee and Badin, North Carolina. The Tennessee smelter also purchases power from the Tennessee Valley Authority under a contract that extends to 2010. With the Badin smelter idled, power generated from APGI’s Yadkin system is largely being sold to an affiliate, Alcoa Power Marketing, Inc.

In the Northeast, the purchased power contracts for the Massena and St. Lawrence, New York smelters expire not earlier than June 30, 2013, following their extension for 10 years upon New York Power Authority having relicensed one of its hydroelectric projects. The company, however, may terminate either of these contracts with one year’s notice.

The Deschambault and Bécancour smelters located in Quebec purchase electricity under long-term contracts with Hydro-Quebec that expire in 2014, subject to extension provisions. The smelter located in Baie Comeau, Quebec purchases approximately 65% of its power needs under a long-term contract with Hydro-Quebec that expires in 2014 and receives the rest of its power needs from a 40%-owned hydroelectric generating company, Manicouagan Power Company.

The Eastalco smelter located in Frederick, Maryland and the Mt. Holly smelter in South Carolina purchase electricity under contracts that expire December 31, 2005 and December 31, 2015, respectively, subject to certain extension provisions.

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