Sec Form 4 Filing - Case Gregory C @ AON CORP - 2012-03-15

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Case Gregory C 2. Issuer Name and Ticker or Trading Symbol AON CORP [ AON] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) President & CEO (Last) (First) (Middle) AON CORPORATION - CORPORATE LAW DEPT, 200 EAST RANDOLPH STREET, 8TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 03/15/2012 (Street) CHICAGO, IL60601 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 02/29/2012 G V 5,000 D $ 0 559,198 D Common Stock 03/15/2012 M( 1 ) 82,364 A $ 41.365 641,562 D Common Stock 03/15/2012 S( 1 ) 82,364 D $ 48.95 ( 2 ) 559,198 D Common Stock 03/16/2012 M( 1 ) 8,203 A $ 41.365 567,401 D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Employee Stock Option (Right to Buy) $ 41.365 03/15/2012 M( 1 ) 82,364 ( 3 ) 03/17/2012 Common Stock 82,364 ( 4 ) 8,203 D Employee Stock Option (Right to Buy) $ 41.365 03/16/2012 M( 1 ) 8,203 ( 3 ) 03/17/2012 Common Stock 8,203 ( 4 ) 0 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Case Gregory C AON CORPORATION - CORPORATE LAW DEPT 200 EAST RANDOLPH STREET, 8TH FLOOR CHICAGO, IL60601 X President & CEO Signatures /s/ Matthew M. Rice - Matthew M. Rice pursuant to a power of attorney from Gregory C. Case 03/19/2012 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The transactions reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on February 8, 2012. ( 2 )The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $48.775 to $49.09, inclusive. The reporting person undertakes to provide to Aon Corporation, or the Staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. ( 3 )Vesting occurred in accordance with the terms of the Aon Stock Incentive Plan as follows: 33.34% of the options vested on the first anniversary of the date of grant, and 33.33% of the options vested on each of the second and third anniversaries of the date of grant. ( 4 )Stock option granted pursuant to the Aon Stock Incentive Plan.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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