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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
 Nlnip7GV0yvZHlTrbwGquPs3dkcKI+FQKQaoUNd0AmAK94ZTlJEdcm2iF40oF3zE
 f6S2lgTu9p7b3HRHybK9rQ==

<SEC-DOCUMENT>0001179110-05-021857.txt : 20051128
<SEC-HEADER>0001179110-05-021857.hdr.sgml : 20051128
<ACCEPTANCE-DATETIME>20051128172623
ACCESSION NUMBER:		0001179110-05-021857
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20051123
FILED AS OF DATE:		20051128
DATE AS OF CHANGE:		20051128

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			MAHONEY DENNIS L
		CENTRAL INDEX KEY:			0001269438

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-07933
		FILM NUMBER:		051229189

	BUSINESS ADDRESS:	
		STREET 1:		AON CORP.
		STREET 2:		200 EAST RANDOLPH STREET
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60601

	FORMER NAME:	
		FORMER CONFORMED NAME:	MOHONEY DENNIS L
		DATE OF NAME CHANGE:	20031106

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			AON CORP
		CENTRAL INDEX KEY:			0000315293
		STANDARD INDUSTRIAL CLASSIFICATION:	ACCIDENT & HEALTH INSURANCE [6321]
		IRS NUMBER:				363051915
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		200 EAST RANDOLPH STREET
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60601
		BUSINESS PHONE:		3123811000

	MAIL ADDRESS:	
		STREET 1:		200 EAST RANDOLPH STREET
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60601

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	COMBINED INTERNATIONAL CORP
		DATE OF NAME CHANGE:	19870504
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>FORM 4 -
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-11-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000315293</issuerCik>
        <issuerName>AON CORP</issuerName>
        <issuerTradingSymbol>AOC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001269438</rptOwnerCik>
            <rptOwnerName>MAHONEY DENNIS L</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>AON CORPORATION - CORPORATE LAW DEPT</rptOwnerStreet1>
            <rptOwnerStreet2>200 EAST RANDOLPH STREET, 8TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60601</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Aon Limited - Chairman/CEO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2005-11-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>7080</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>36.43</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Includes 2,655 shares of common stock received by the reporting person in January 2004 in connection with the vesting of a restricted stock award that were inadvertently omitted from previous filings.  The grant of the restricted stock award was previously reported in Table II of the reporting person's initial statement of beneficial ownership on Form 3 as a derivative security.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Jennifer L. Kraft - Jennifer L. Kraft pursuant to a power of attorney from Dennis L. Mahoney</signatureName>
        <signatureDate>2005-11-28</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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