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): December 13, 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:

Mr. Steinke, 51, joined Ameren in 2002 and has served in various accounting, financial reporting and investor relations capacities of the Ameren companies. In December 2007, Mr. Steinke was elected Vice President and Controller of Ameren and certain subsidiaries, and has served in such position since January 1, 2008. In recognition of Mr. Steinke’s significant increase in management and oversight responsibilities, the Human Resources Committee (the “Committee”) of the Board approved an increase to Mr. Steinke’s base salary from $253,900 to $300,000, effective January 1, 2013. In addition, Mr. Steinke’s target short-term incentive opportunity under Ameren’s Executive Incentive Plan (“EIP”) and target long-term incentive opportunity under Ameren’s Performance Share Unit Program (“PSUP”), each as described under Item 5.02(e) below, has been increased, effective January 1, 2013, from 45% to 50% of his base salary and from 80% to 100% of his base salary, respectively. Mr. Steinke’s actual payments under these programs may vary from the targets based upon Ameren’s performance and his performance in accordance with the formulas and methodologies in the EIP and PSUP.

For 2013, a target award under the 2013 EIP was established for each Named Executive Officer as a percent of 2013 base salary as shown below. For purposes of the 2013 EIP, 2013 base salary is salary at the end of the plan year (except as otherwise provided in the 2013 EIP).

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Under the 2013 EIP, EPS (as determined in accordance with generally accepted accounting principles) is the metric used to establish 90% of award opportunities. In addition, under the 2013 EIP, 10% of award opportunities will be attributed to a safety performance measure, lost workday away cases (“LWA”). The range of EPS and LWA achievement levels for the 2013 EIP (threshold, target and maximum) will be established by the Committee in February 2013. EPS achievement levels may be adjusted to include or exclude specified items of an unusual nature or non-operating or significant events not anticipated in the business plan when EPS achievement levels were established, as determined by the Committee at its sole discretion and as permitted by the Ameren Corporation 2006 Omnibus Incentive Compensation Plan (the “Omnibus Plan”). In the event Ameren EPS or LWA is below the respective threshold achievement level, as determined by the Committee, no award will be attributed to EPS or LWA, as applicable.

The 2013 EIP award based on achievement of 2013 Ameren EPS and LWA (the “base award”) for each Named Executive Officer may be adjusted up or down by up to 50%, with the ability to pay zero for poor or non-performance (the “individual performance modifier”) based on the Named Executive Officer’s individual contributions and performance during the year, in the Committee’s discretion. The individual performance modifier will take into consideration the Named Executive Officer’s performance on key performance variables, including leadership, business results, customer satisfaction, reliability, plant availability, safety and/or other performance metrics, as applicable and as determined by the Committee. The actual individual incentive payout, determined by modifying the base award by the individual performance modifier, is capped at 200% of target short-term incentive opportunity.

On December 13, 2012, the Committee also authorized the issuance pursuant to the PSUP under the Omnibus Plan of performance share unit awards for each of the Named Executive Officers identified on Exhibit 99.1 attached hereto. The target number of performance share units to be issued to each such Named Executive Officer for 2013 pursuant to the Omnibus Plan will be determined as of January 1, 2013 in accordance with the formula set forth in Exhibit 99.1 attached hereto.

Each performance share unit represents the right to receive a share of Ameren’s common stock assuming certain performance criteria are achieved. The actual number of performance share units which may be earned will vary from 0% to 200% of the target

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number of performance share units granted to each Named Executive Officer, based primarily on Ameren’s 2013-2015 three-year total shareholder return (“TSR”) relative to a utility peer group and continued employment during such three-year period (the “Performance Period”). In the event Ameren’s TSR relative to utility peers during the Performance Period is below the 30th percentile, no performance share units will be earned unless the three-year average Ameren EPS (as determined in accordance with generally accepted accounting principles), subject to adjustment to include or exclude specified items of an unusual nature or non-operating or significant events not anticipated in the business plan when EPS achievement levels were established as determined by the Committee in its sole discretion as permitted by the Omnibus Plan, reaches or exceeds the average of the Ameren Executive Incentive Plan threshold levels for 2013, 2014 and 2015.

Dividends on performance share units will accrue and be reinvested into additional performance share units throughout the Performance Period on the shares ultimately earned. Because these performance share units will be earned only if performance goals over the Performance Period are attained, the amounts, if any, that will be payable to the Named Executive Officers pursuant to the performance share unit awards described above are not determinable at this time. The Committee has the ability to amend the terms of the performance share unit awards, including the performance criteria, to the extent not adverse to the holders of an award.

The form of performance share unit award agreement for 2013 is attached as Exhibit 10.2 and is incorporated herein by reference.

The award agreements between Ameren and each of the Named Executive Officers provide that upon the occurrence of a “Change of Control” if Ameren continues to exist and its common stock is traded on the New York Stock Exchange (“NYSE”) or NASDAQ Stock Market (“NASDAQ”) (i) voluntary terminations of employment prior to the end of the performance period or terminations for “Cause” at any time prior to pay out of shares result in forfeiture of awards and (ii) a “qualifying termination” (as defined in the award agreement) of employment during the performance period does not change the manner in which awards are earned and become vested on December 31, 2015 and such awards will be paid in shares of Ameren common stock on January 1, 2016 or as soon as practicable thereafter (except as otherwise provided in the awards).

The award agreements referenced above also provide that upon a “Change of Control” which occurs on or before December 31, 2015, if Ameren ceases to exist or its common stock is no longer traded on the NYSE or NASDAQ (i) the target number of performance share units together with accrued dividends thereon will be converted into nonqualified deferred compensation which will accrue interest at the prime rate as provided in the award agreement and, assuming continued employment, such amount will be paid out on January 1, 2016 or as soon as practicable thereafter, (ii) voluntary terminations of employment or terminations for “Cause” result in forfeiture of the amounts described above in (i) of this paragraph and (iii) qualifying terminations of employment result in an

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immediate payment of the amounts described above in (i) of this paragraph (except as otherwise provided in the awards).

The terms “Change of Control” and “Cause” have the meanings given them in the Second Amended and Restated Ameren Corporation Change of Control Severance Plan, a copy of which was filed as Exhibit 10.37 to Ameren’s Annual Report filed on Form 10-K for the 2008 fiscal year.

The complete copy of the Certificate Amendment is included as Exhibit 3.1(i).

Also on December 14, 2012, the Board amended Ameren’s By-Laws, effective December 14, 2012. The amendments relate to certain indemnification and advancement of expenses in Article IV of Ameren’s By-Laws. The principal changes to Article IV of Ameren’s By-Laws:

The complete copy of the Ameren By-Laws, as amended, is included as Exhibit 3.1(ii).

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(d) Exhibits

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SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, Ameren has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

Date: December 18, 2012

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Exhibit Index

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