UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of report (Date of earliest event reported): September 20, 2012

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Reference is made to Note 9. Commitments and Contingencies to our financial statements under Part I, Item 1. Financial Statements (Unaudited) and Outlook under Part I, Item 2. Management’s Discussion and Analysis of Financial Condition and Results of Operations in the Form 10-Q for the quarter ended June 30, 2012, of registrant Ameren Corporation (“Ameren”) and its registrant subsidiary, Ameren Energy Generating Company (“Genco”), for a discussion of Ameren Energy Resources Company’s (“AER”) request to the Illinois Pollution Control Board (“IPCB”) for a variance to extend compliance dates for sulfur dioxide (“SO₂”) emission levels contained in the Illinois Multi-Pollutant Standards (“MPS”).

On September 20, 2012, the IPCB granted AER a variance to extend compliance dates for SO₂ emission levels contained in the MPS through December 31, 2019, subject to certain conditions as described below. AER had requested the variance extend through December 31, 2020. The IPCB also approved AER’s proposed plan to restrict its SO₂ emissions through 2014 to levels lower than those required by the existing MPS to offset any environmental impact from the variance. The order also requires AER to comply with a schedule of deadlines set forth in the order for completion of various aspects of the installation and completion of a scrubber project at Genco’s Newton energy center; the first milestone relates to the completion of engineering design by July 2015 while the last deadline relates to major equipment components being placed into final position on or before September 1, 2019. The order also requires AER to refrain from operating the Meredosia and Hutsonville energy centers through December 31, 2020; however, this restriction does not impact Ameren’s and Genco’s ability to make the Meredosia energy center available for any parties that may be interested in repowering one of its units to create an oxy-fuel combustion coal-fired energy center designed for permanent carbon dioxide capture and storage. AER is evaluating the order granting the variance and has to decide whether to accept the terms and conditions of the variance set forth in the order within 45 days from September 20, 2012.

The IPCB’s ruling gives AER additional time for economic recovery and related power price improvements necessary to support scrubber installations and other pollution controls at some of AER’s energy centers. To comply with the existing MPS and other air emissions laws and regulations, Genco and AmerenEnergy Resources Generating Company (“AERG”), which are each subsidiaries of AER, are installing equipment designed to reduce their emissions of mercury, nitrogen oxide (“NO_x”), and SO₂. Genco and AERG have installed a total of three scrubbers at two energy centers. Two additional scrubbers are being constructed at Genco’s Newton energy center. The timing of the installation of the scrubbers at Genco’s Newton energy center, as well as precipitator upgrades at AERG’s E.D. Edwards energy center, has been delayed. Ameren’s merchant generation business segment and Genco will continue to review and adjust their compliance plans in light of evolving outlooks for power and capacity prices, delivered fuel costs, environmental standards and compliance technologies, among other factors.

Forward-looking Statements

Statements in this report not based on historical facts are considered “forward-looking” and, accordingly, involve risks and uncertainties that could cause actual results to differ materially from those discussed. Although such forward-looking statements have been made in good faith and are based on reasonable assumptions, there is no assurance that the expected results will be achieved. These statements include (without limitation) statements as to future expectations, beliefs, plans, strategies, objectives, events, conditions, and financial performance. In connection with the “safe harbor” provisions of the Private Securities Litigation Reform Act of 1995, we are providing this cautionary statement to identify important factors that could cause actual results to differ materially from those anticipated. The following factors, in addition to those discussed under Risk Factors in Ameren’s Form 10-K for the year ended December 31, 2011, and elsewhere in this report and in our other filings with the Securities and Exchange Commission, could cause actual results to differ materially from management expectations suggested in such forward-looking statements:

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Given these uncertainties, undue reliance should not be placed on these forward-looking statements. Except to the extent required by the federal securities laws, we undertake no obligation to update or revise publicly any forward-looking statements to reflect new information or future events.

This combined Form 8-K is being filed separately by Ameren Corporation and Ameren Energy Generating Company (each a “registrant”). Information contained herein relating to any individual registrant has been filed by such registrant on its own behalf. No registrant makes any representation as to information relating to any other registrant.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, each registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized. The signature for each undersigned company shall be deemed to relate only to matters having reference to such company or its subsidiaries.

Date: September 21, 2012