Exhibit 5

                                December 3, 2001

Aon Corporation
200 East Randolph Street
Chicago, Illinois 60601


     Re:  $750,000,000 OF DEBT SECURITIES, PREFERRED STOCK, COMMON STOCK, SHARE
          PURCHASE CONTRACTS AND SHARE PURCHASE UNITS

Ladies and Gentlemen:

     I am Senior Counsel and Assistant Secretary of Aon Corporation (the
"Company"). I refer to the Registration Statement on Form S-3 (the "Registration
Statement") being filed by the Company, with the Securities and Exchange
Commission (the "SEC") under the Securities Act of 1933, as amended (the
"Securities Act"), relating to the shelf registration of $750,000,000 of the
Company's debt securities (the "Debt Securities"), preferred stock, par value
$1.00 per share ("Preferred Stock"), common stock, par value $1.00 per share
("Common Stock"), share purchase contracts obligating the holders to purchase
from the Company a specified number of Common Stock ("Share Purchase Contracts")
which may be issued separately or as part of share purchase units, each
consisting of a Share Purchase Contract and of a beneficial interest in the
Company's Debt Securities, Preferred Stock or debt obligations of third parties
securing the holder's obligations to purchase Common Shares under the Share
Purchase Contracts ("Share Purchase Units"). The Debt Securities are to be
issued under an Indenture dated as of September 15, 1992 between the Company and
The Bank of New York, as successor trustee (the "Senior Debt Indenture"), and an
Indenture to be dated as of a date before the first issuance of subordinated
debt securities between the Company and U.S. Bank Trust National Association, as
trustee (the "Subordinated Debt Indenture" and together with the Senior Debt
Indenture, the "Indentures"). The Preferred Stock will be issued pursuant to a
Certificate of Designation relating to a particular series of Preferred Stock.

     I am familiar with the proceedings to date with respect to the proposed
issuance and sale of the Debt Securities, the Preferred Stock, the Common Stock,
the Share Purchase Contracts and the Share Purchase Units and have examined
such records, documents and questions of law, and satisfied myself as to such
matters of fact, as I have considered relevant and necessary as a basis for this
opinion.

     In rendering the opinions expressed below, I have assumed the authenticity
of all documents submitted to me as originals and the conformity to the
originals of all documents submitted to me as copies. In addition, I have
assumed and have not verified the accuracy as to factual matters of each
document I reviewed.

     Based on the foregoing, I am of the opinion that:




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December 3, 2001
Page 2


1.   The Company is duly incorporated and validly existing under the laws of the
     State of Delaware.

2.   The Company has corporate power and authority to authorize and sell the
     Debt Securities, the Preferred Stock, the Common Stock, the Share Purchase
     Contracts and the Share Purchase Units.

3.   Each series of Debt Securities will be legally issued and binding
     obligations of the Company (except to the extent enforceability may be
     limited by applicable bankruptcy, insolvency, reorganization, moratorium,
     fraudulent transfer or other similar laws affecting the enforcement of
     creditors' rights generally and by the effect of general principles of
     equity, regardless of whether enforceability is considered in a proceeding
     in equity or at law) when (i) the Registration Statement, as finally
     amended (including any necessary post-effective amendments), shall have
     become effective under the Securities Act and the Indentures (including any
     necessary supplemental indentures) shall have been qualified under the
     Trust Indenture Act of 1939, as amended; (ii) a Prospectus Supplement with
     respect to such series of Debt Securities shall have been filed with the
     SEC pursuant to Rule 424 under the Securities Act; (iii) the Company's
     board of directors or a duly authorized committee thereof shall have duly
     adopted final resolutions authorizing the issuance and sale of such series
     of Debt Securities as contemplated by the Registration Statement and the
     Indentures; (iv) such series of Debt Securities shall have been duly
     executed and authenticated as provided in the Indentures and such
     resolutions and shall have been duly delivered to the purchasers thereof
     against payment of the agreed consideration therefor; (v) any consents
     required pursuant to the Company's credit facilities shall have been
     obtained; and (vi) in the case of any Debt Securities issued under the
     Subordinated Debt Indenture, such Subordinated Debt Indenture shall have
     been duly executed and delivered by the Company and U.S. Bank Trust
     National Association, as trustee.

4.   Each series of Preferred Stock will be validly issued, fully paid and
     nonassessable when (i) the Registration Statement, as finally amended
     (including any necessary post-effective amendments), shall have become
     effective under the Securities Act; (ii) a Prospectus Supplement with
     respect to the sale of such series of Preferred Stock shall have been filed
     with the SEC pursuant to Rule 424 under the Securities Act; (iii) the
     Company's board of directors or a duly authorized committee thereof shall
     have duly adopted resolutions specifying the terms and conditions of such
     series of Preferred Stock and authorizing its issuance; (iv) the Company
     shall have filed with the Delaware Secretary of State a certificate of
     designation with respect to such series of Preferred Stock; and (v)
     certificates representing such series of Preferred Stock shall have been
     duly executed, countersigned and registered and duly delivered to the
     purchasers thereof against payment therefor in the manner described in the
     Registration Statement.

5.   The shares of Common Stock will be validly issued, fully paid and
     nonassessable when: (i) the Registration Statement, as finally amended
     (including any necessary post-effective amendments), shall have become
     effective under the Securities Act; (ii) a Prospectus




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December 3, 2001
Page 3


     Supplement with respect to the sale of such Common Stock shall have been
     filed with the SEC pursuant to Rule 424 under the Securities Act; (iii) the
     Company's board of directors or a duly authorized committee thereof shall
     have duly adopted final resolutions authorizing the issuance and sale of
     the Common Stock; and (iv) certificates representing the Common Stock shall
     have been duly executed, countersigned and registered and duly delivered to
     the purchasers thereof against payment therefor in the manner described in
     the Registration Statement.

6.   The Share Purchase Contracts and/or the Share Purchase Units will be
     legally issued and binding obligations of the Company (except to the extent
     enforceability may be limited by applicable bankruptcy, insolvency,
     reorganization, moratorium, fraudulent transfer or other similar laws
     affecting the enforcement of creditors' rights generally and by the effect
     of general principles of equity, regardless of whether enforceability is
     considered in a proceeding in equity or at law) when (i) the Registration
     Statement, as finally amended (including any necessary post-effective
     amendments), shall have become effective under the Securities Act; (ii) a
     Prospectus Supplement with respect to the sale of such Share Purchase
     Contracts and/or Share Purchase Units shall have been filed with the SEC
     pursuant to Rule 424 under the Securities Act; (iii) the Company's board of
     directors or a duly authorized committee thereof shall have duly adopted
     resolutions specifying the terms and conditions of such Share Purchase
     Contracts and/or Share Purchase Units and authorizing such issuance; (iv)
     any Common Stock, Preferred Stock, Debt Securities and/or debt obligations
     of third parties issued as a security for the relevant Share Purchase
     Contract and/or as part of the relevant Share Purchase Units are duly
     authorized and validly issued; and (v) certificates representing such Share
     Purchase Contracts and/or Share Purchase Units shall have been duly
     executed, countersigned and registered and duly delivered to the purchasers
     thereof against payment therefor in the manner described in the
     Registration Statement.

     This opinion letter is limited to the General Corporation Law of the State
of Delaware, the laws of the State of Illinois and the federal laws of the
United States of America.

     For the purposes of this opinion letter, I have assumed that, at the time
of the issuance, sale and delivery of each series of Debt Securities and
Preferred Stock, each issue of Common Stock, Share Purchase Contracts and Share
Purchase Units, as the case may be: (i) the authorization thereof by the
Company will not have been modified or rescinded, and there will not have
occurred any change in law affecting the validity, legally binding character or
enforceability thereof; (ii) in the case of the Debt Securities, the Indentures
will not have been modified or amended; (iii) in the case of the issue of any
series of Preferred Stock or the issue of Common Stock, the terms and conditions
of such Preferred Stock or Common Stock, as the case may be, will be as
expressly contemplated by the Registration Statement; and (iv) the Second
Restated Certificate of Incorporation, as amended, and the By-Laws of the
Company, as currently in effect, will not have been modified or amended and will
be in full force and effect.

     I do not find it necessary for the purposes of this opinion letter to
cover, and accordingly I express no opinion as to, the application of the
securities or blue sky laws of the




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December 3, 2001
Page 4

various states or the District of Columbia to sales of the Debt Securities, the
Preferred Stock, the Common Stock, the Share Purchase Contracts or the Share
Purchase Units.

     I hereby consent to the filing of this opinion letter as an Exhibit to the
Registration Statement and to all references to me under the caption "Validity
of Securities" in the Prospectus forming a part of the Registration Statement.
In giving such consent, I do not thereby admit that I am within the category of
persons whose consent is required by Section 7 of the Securities Act or the
related rules and regulations promulgated by the SEC.

                                       Very truly yours,

                                       /s/ Richard E. Barry