S-3

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As filed with the Securities and Exchange Commission on December 3, 2001

Registration No. 333-    

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM S-3

Registration Statement under the Securities Act of 1933

Aon CORPORATION
(Exact Name of Registrant as Specified in its Charter)

Delaware
(State or Other Jurisdiction of Incorporation or Organization) 36-3051915
(I.R.S. Employer Identification No.)

200 East Randolph Street, Chicago, Illinois 60601, (312) 381-1000
(Address, including zip code, and telephone number, including area code, of Registrant's principal executive offices)

Raymond I. Skilling, Executive Vice President and Chief Counsel, Aon Corporation
200 East Randolph Street, Chicago, Illinois 60601, (312) 381-1000
(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copy of communications to:
Frederick C. Lowinger and Paul L. Choi, Sidley Austin Brown & Wood
Bank One Plaza, 10 South Dearborn Street, Chicago, Illinois 60603, (312) 853-7000

Approximate date of commencement of proposed sale to the public: From time to time after this Registration Statement becomes effective as determined by market conditions.

  If the only securities being registered on this form are being offered pursuant to dividend or interest reinvestment plans, please check the following box. / /

  If any of the securities being registered on this form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box. /x/

  If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. / /

  If this form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. / /

  If delivery of the prospectus is expected to be made pursuant to Rule 434, please check the following box. / /

CALCULATION OF REGISTRATION FEE

Title of Each Class of Securities to be Registered	Amount to be Registered	Proposed Maximum Offering Price Per Unit	Proposed Maximum Aggregate Offering Price(2)(3)	Amount of Registration Fee
Debt securities (4)		(1)
Preferred stock, par value $1.00 per share (5)		(1)
Common stock, par value $1.00 per share (6)		(1)
Share Purchase Contracts		(1)
Share Purchase Units (7)		(1)
Total (8)	$750,000,000	100%	$750,000,000	$179,250

(1)

The proposed maximum offering price per unit will be determined at various times by the Registrant in connection with the issuance by the Registrant of the securities registered hereunder.

(2)

The proposed maximum aggregate offering price has been estimated only for the purpose of calculating the registration fee pursuant to Rule 457 under the Securities Act of 1933, as amended. The aggregate public offering price of the debt securities, preferred stock, common stock, share purchase contracts and share purchase units registered hereby will not exceed $750,000,000 or the equivalent thereof in one or more foreign currencies, foreign currency units or composite currencies.

(3)

Exclusive of accrued interest, distributions and dividends, if any.

(4)

Subject to note (8) below, there is being registered hereunder an indeterminate principal amount of debt securities of the Registrant as may be sold at various times. If any debt securities are issued at an original issue discount, such greater aggregate principal amount is registered hereby, such that the aggregate public offering price thereof does not exceed $750,000,000.

(5)

Subject to note (8) below, there is being registered hereunder an indeterminate number of shares of preferred stock of the Registrant as at various times may be issued at indeterminate prices.

(6)

Subject to note (8) below, there is being registered hereunder an indeterminate number of shares of common stock of the Registrant as at various times may be issued at indeterminate prices. This Registration Statement also covers such indeterminate number of shares of common stock of the Registrant, if any, (i) as shall be issuable or deliverable upon conversion of any debt securities or preferred stock which are convertible into such common stock, (ii) as may be required for delivery upon conversion of any such convertible debt securities or preferred stock as a result of any anti-dilution provisions thereof and (iii) as shall be issuable or deliverable in connection with the share purchase contracts and share purchase units.

(7)

Each share purchase unit consists of, among other things, (a) a share purchase contract under which the holder, upon settlement, will purchase an indeterminate number of shares of the Registrant's common stock and (b) a beneficial interest in debt securities, preferred securities or debt obligations of third parties purchased with the proceeds from the sale of the share purchase units. Each beneficial interest will be pledged to secure the obligation of such holder to purchase such shares of Registrant's common stock. No separate consideration will be received for the share purchase contract or the related beneficial interests.

(8)

In no event will the aggregate offering price of all securities issued at various times pursuant to this Registration Statement exceed $750,000,000 or the equivalent thereof in one or more foreign currencies, foreign currency units or composite currencies.

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, or until the Registration Statement shall become effective on such date as the Commission, acting pursuant to said Section 8(a), may determine.

Subject To Completion, Dated December 3, 2001

The information in this prospectus is not complete and may be changed. We may not sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell these securities and it is not soliciting an offer to buy these securities in any state where the offer or sale is not permitted.

PROSPECTUS

Aon Corporation

Debt Securities
Preferred Stock
Common Stock
Share Purchase Contracts
Share Purchase Units

    We will describe the specific terms of these securities in one or more supplements to this prospectus. You should read this prospectus and the applicable supplements carefully before you invest.

    Our executive offices are located at 200 East Randolph Street, Chicago, Illinois 60601, and our telephone number is (312) 381-1000.

    Our common stock is listed on the New York Stock Exchange under the symbol "AOC."

    Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

    We may offer these securities in any of the following ways:

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directly to purchasers;



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through agents;



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through dealers; or



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through one or more underwriters or a syndicate of underwriters in an underwritten offering.

    Additional information on our plan of distribution can be found inside under "Plan of Distribution." We will describe the plan of distribution for any securities in the applicable prospectus supplements.

The date of this Prospectus is         , 2001.

TABLE OF CONTENTS

About This Prospectus	2
Where You Can Find More Information	2
The Company	4
Use Of Proceeds	4
Ratios	4
Description of Debt Securities	5
Description of Preferred Stock and Common Stock	14
Description of the Share Purchase Contracts and the Share Purchase Units	16
Plan of Distribution	17
Validity of Securities	18
Experts	18

ABOUT THIS PROSPECTUS

    This prospectus is part of a registration statement (No. 333-      ) that we filed with the Securities and Exchange Commission utilizing a "shelf" registration process. Under this shelf process, we may offer up to $750,000,000 aggregate public offering price of the debt securities, preferred stock, common stock, share purchase contracts and share purchase units described in this prospectus in one or more offerings. In this prospectus we will refer to the debt securities, preferred stock, common stock, share purchase contracts and share purchase units collectively as the "securities." This prospectus provides you with a general description of the securities we may offer. Each time we offer securities, we will provide you with a prospectus supplement and, if applicable, a pricing supplement. The prospectus supplement and any applicable pricing supplement will describe the specific terms of the securities being offered. The prospectus supplement and any applicable pricing supplement may also add, update or change the information in this prospectus. Please carefully read this prospectus, the applicable prospectus supplement and any applicable pricing supplement, together with the information contained in the documents referred to under the heading "Where You Can Find More Information."

WHERE YOU CAN FIND MORE INFORMATION

    We file annual, quarterly and special reports, proxy statements, and other information with the SEC. You may read and copy any document we file with the SEC at the SEC's Public Reference Room at 450 Fifth Street, N.W., Washington, D.C. 20549 or at the SEC's regional offices located at Citicorp Center, 500 West Madison Street, Suite 1400, Chicago, Illinois 60661-2511 and 233 Broadway, New York, New York 10005. You may obtain further information on the operation of the Public Reference Room by calling the SEC at 1-800-SEC-0330. Our SEC filings are also available to the public over the Internet at the SEC's Web site at http://www.sec.gov. In addition, you may inspect our SEC filings at the offices of the New York Stock Exchange, 20 Broad Street, New York, New York 10005, the Chicago Stock Exchange, 440 South LaSalle Street, Chicago, Illinois 60605 and the London Stock Exchange, Old Broad Street, London, England EC2N 1HP. You may find additional information about Aon Corporation and its subsidiaries at our Web site at http://www.aon.com. The information on our web site is not a part of this prospectus.

    The SEC allows us to "incorporate by reference" into this prospectus the information we file with the SEC, which means that we can disclose important information to you by referring you to those documents. Any information referenced this way is considered to be part of this prospectus, and any information that we file later with the SEC will automatically update and supersede this information. We incorporate by reference the following documents that we have filed with the SEC and any future filings that we make with the SEC under Sections 13(a), 13(c), 14, or 15(d) of the Securities Exchange Act of 1934 after the date of the initial registration statement and before the effectiveness of this

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registration statement and after the effectiveness of this registration statement until we complete our sale of the securities to the public:

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Annual Report on Form 10-K for the year ended December 31, 2000;



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Proxy statement for the 2001 Annual Meeting of Stockholders filed on March 5, 2001;



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Quarterly Reports on Form 10-Q for the quarters ended March 31, 2001, June 30, 2001 and September 30, 2001 (as amended on November 28, 2001);



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Current Reports on Form 8-K dated April 24, 2001, November 8, 2001 and December 3, 2001; and



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The description of Aon Corporation's common stock contained in Item 12 of the registration statement on Form 10 filed on February 19, 1980 (when we were called Combined International Corporation), and any amendment or report which we have filed (or will file after the date of this prospectus and prior to the termination of this offering) for the purpose of updating such description, including Aon Corporation's Current Report on Form 8-K dated April 23, 1987 and Current Report on Form 8-K dated May 9, 2000.

    This prospectus is part of a registration statement we have filed with the SEC relating to the securities. As permitted by SEC rules, this prospectus does not contain all of the information included in the registration statement and the accompanying exhibits and schedules we file with the SEC. You may refer to the registration statement and the exhibits and schedules for more information about us and our securities. The registration statement and exhibits and schedules are also available at the SEC's Public Reference Room or through its Web site.

    You may obtain a copy of these filings, at no cost, by writing to or telephoning us at the following address:

Aon Corporation
200 East Randolph Street
Chicago, Illinois 60601
Telephone (312) 381-1000
Attention: Corporate Secretary

    You should rely only on the information incorporated by reference or provided in this prospectus and the applicable prospectus supplement and in any pricing supplement. We have not authorized anyone else to provide you with different information. You should not assume that the information in this prospectus, any applicable prospectus supplement or any pricing supplement is accurate as of any date other than the date on the cover of the applicable document. We are not making an offer of the securities in any state where the offer or sale is not permitted.

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THE COMPANY

    Aon Corporation is a holding company that is comprised of a family of insurance brokerage, consulting and insurance underwriting subsidiaries. Through our insurance brokerage and other services and consulting operations, we offer commercial insurance brokerage, alternative risk solutions, risk management, employee benefit and human resources consulting and managing general underwriting services. In addition, our insurance underwriting businesses provide a variety of insurance products, including supplemental accident and health coverage, traditional life insurance and extended warranties. Our revenues were $7.4 billion in 2000. Based on 2000 insurance brokerage and consulting revenues, we are the second largest insurance brokerage company in the world. The mailing address of our principal executive offices is 200 East Randolph Street, Chicago, Illinois 60601, and the telephone number of our principal executive offices is (312) 381-1000.

USE OF PROCEEDS

    Unless we state otherwise in the applicable prospectus supplement, we expect to use the net proceeds from the sale of the securities for general corporate purposes, including securities repurchase programs, capital expenditures, working capital, capital contributions to our insurance subsidiaries, repayment or reduction of long-term and short-term debt and the financing of acquisitions. We may invest funds that we do not immediately require in short-term marketable securities.

RATIOS

    Our ratios of earnings to fixed charges and of earnings to combined fixed charges and preferred stock dividends for each of the periods indicated are as follows:

	Nine Months Ended September 30,		Year Ended December 31,
	2001	2000	2000	1999	1998	1997	1996
Ratio of earnings to fixed charges(1)	2.8x	6.0x	5.4x	5.1x	7.6x	5.6x	6.7x
Ratio of earnings to combined fixed charges and preferred stock dividends(2)	2.0x	4.3x	4.0x	3.5x	5.1x	3.3x	4.9x

(1)

Reflects the inclusion in income from continuing operations before provision for income taxes and minority interest of unusual charges of $53 million related to the World Trade Center tragedy and special charges of $218 million for the nine months ended September 30, 2001. Reflects the inclusion in income from continuing operations before provision for income taxes and minority interest of special charges of $82 million, $313 million, $172 million and $90 million for the years ended December 31, 2000, 1999, 1997 and 1996, respectively.

(2)

Included in total fixed charges and preferred stock dividends are $49 million for the nine months ended September 30, 2001 and 2000, $66 million for the years ended December 31, 2000, 1999 and 1998, and $64 million for the year ended December 31, 1997, of pretax distributions on the 8.205% mandatorily redeemable preferred capital securities which are classified as "minority interest" on the condensed consolidated statements on income.

    For these ratios, earnings consist of income from continuing operations before provision for income taxes and minority interests and before fixed charges. Fixed charges include interest expense and that portion of rental expense we deem to represent interest. Combined fixed charges and preferred stock dividends include preferred stock dividend requirements, interest expense and that portion of rental expense we deem to represent interest. Preferred stock dividends consist of the pre-tax earnings required to pay dividends on all preferred stock. Our earnings, fixed charges and preferred stock dividends include the earnings, fixed charges and preferred stock dividends of Aon Corporation and its subsidiaries considered as one enterprise.

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DESCRIPTION OF DEBT SECURITIES

    This prospectus describes certain general terms and provisions of our debt securities. When we offer to sell a particular series of debt securities, we will describe the specific terms of the series in a supplement to this prospectus. We will also indicate in the applicable prospectus supplement the extent to which the general terms and provisions described in this prospectus apply to a particular series of debt securities.

    The debt securities we may offer pursuant to this prospectus will be unsecured obligations of Aon Corporation and will be either senior or subordinated debt. We will issue senior debt under an indenture dated as of September 15, 1992, as may be supplemented, between us and The Bank of New York, as the successor senior indenture trustee. We will issue subordinated debt under an indenture to be dated as of a date before the first issuance of subordinated debt, as may be supplemented, between us and U.S. Bank Trust National Association, as the subordinated indenture trustee. The senior debt indenture and the subordinated debt indenture are sometimes referred to in this prospectus individually as an "indenture" and collectively as the "indentures" and the senior indenture trustee and the subordinated indenture trustee are sometimes referred to in this prospectus individually as a "trustee." We have summarized selected provisions of the indentures below. This is a summary and is not complete. It does not describe certain exceptions and qualifications contained in the indentures or the debt securities. If you would like more information on the provisions of either of the indentures, you should read the more detailed provisions of the applicable indenture, both of which have been incorporated by reference as exhibits to the registration statement. In the summary, we have included parenthetical references to the section numbers of the applicable indenture so that you can easily locate those provisions.

General

    The debt securities will be unsecured obligations of Aon Corporation. The indentures do not limit the amount of debt securities that we may issue under them. The indentures provide that we may issue debt securities from time to time in one or more series. We have previously issued debt securities pursuant to the senior debt indenture.

    The debt securities issued under the senior debt indenture will be unsecured obligations and will rank equally with each other and with all of our other unsecured and unsubordinated indebtedness. The debt securities issued under the subordinated debt indenture will be subordinate and junior in right of payment, as more fully described in the subordinated debt indenture, to all of our senior indebtedness. See "—Subordination under the Subordinated Debt Indenture."

    Because we are a holding company, the holders of the debt securities may not receive assets of our subsidiaries in a liquidation or recapitalization until the claims of our subsidiaries' creditors and insurance policyholders (in the case of insurance subsidiaries) are paid, except to the extent that we may have recognized claims against such subsidiaries. In addition, certain regulatory laws limit some of our subsidiaries, including Combined Insurance Company of America, from making payments to us of dividends and on loans and other transfers of funds.

    We will include in a supplement to this prospectus and any pricing supplement the specific terms relating to the debt securities being offered. These terms will include some or all of the following:

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the title of the debt securities and whether the debt securities will be senior or subordinated debt;

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the total principal amount of the debt securities;

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the maturity date or dates of the debt securities;

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the interest rate or rates, if any (which may be fixed or variable) and, if applicable, the method used to calculate the interest rate;

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•

the date or dates from which interest will accrue and on which interest will be payable and the dates used to determine the persons to whom interest will be paid;

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the place or places where principal of, and any premium or interest on, the debt securities will be paid;

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whether (and if so, when and under what terms and conditions) the debt securities may be redeemed by us at our option or at the option of the holders;

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whether there will be a sinking fund;

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if other than United States dollars and denominations of $1,000 or any multiple of $1,000, the currency or currencies or currency unit or currency units and denomination in which the debt securities will be issued;

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if other than the principal amount, the portion of the principal amount of the debt securities that we will pay upon acceleration of the maturity date;

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whether we will issue the debt securities in registered or bearer ("unregistered") form or both;

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if we issue debt securities in unregistered form, any restrictions on the exchange of one form for another and to the offer, sale and delivery of such unregistered securities;

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whether and under what circumstances and conditions we will pay additional amounts on the debt securities held by foreign persons in respect of any tax, assessment or governmental charge imposed on such holders with respect to the debt securities ("additional amounts");

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whether we will issue the debt securities in certificated or book-entry form;

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with respect to subordinated debt securities, whether they will be convertible into shares of common stock and the terms and conditions governing such conversion; and

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any other terms of the series being offered, so long as they are not inconsistent with any provision of the applicable indenture. (Section 2.01)

    If we denominate the purchase price of a series of debt securities in a foreign currency or currencies or a foreign currency unit or units, or if the principal of, any premium and interest on, and any additional amounts with respect to any series of debt securities is payable in a foreign currency or currencies or a foreign currency unit or units, we will describe any special United States federal income tax considerations in the applicable prospectus supplement.

    We will pay principal and any interest, premium and additional amounts in the manner, at the places and subject to the restrictions set forth in the indentures, the debt securities and the applicable prospectus supplement. We will not impose a service charge for any transfer or exchange of debt securities, but we may require payment of a sum sufficient to cover any tax or other governmental charge that may be imposed. (Section 2.05) Unregistered debt securities and any related coupons will be transferable by delivery. (Section 2.05)

    Unless otherwise indicated in the applicable prospectus supplement, we will issue debt securities in fully registered form, without coupons, in denominations of $1,000 or multiples of $1,000. (Sections 2.01 and 2.04)

    We may offer to sell at a substantial discount below their stated principal amount, debt securities bearing no interest or interest at a rate that, at the time of issuance, is below the prevailing market rate. We will describe any special United States federal income tax considerations applicable to any of those discounted debt securities in the applicable prospectus supplement.

    We may offer to sell debt securities in which the principal or interest will be determined by reference to one or more currency exchange rates, commodity prices, equity indices or other factors. The principal amount or payment of interest applicable to such debt securities may be greater than or less than the amount of principal or interest otherwise payable, depending upon the value of the applicable currency, commodity, equity index or other factor on the date on which such principal or

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interest is due. We will set forth in the applicable prospectus supplement information about the methods used to determine the amount of principal or interest payable on any date, the currencies, commodities, equity indices or other factors to which the amount payable on that date is linked and certain additional tax considerations applicable to such debt securities.

    The senior debt securities will contain limits on our ability to incur certain secured indebtedness. See "—Limitation on Liens under Senior Debt Securities." The indentures do not restrict our ability to incur unsecured indebtedness or, subject to the restrictions described in "—Consolidation and Merger," to engage in reorganizations, restructurings, mergers, consolidations or similar transactions that have the effect of increasing our indebtedness. Accordingly, unless we state otherwise in the applicable supplement, the debt securities will not contain any provisions that afford holders protection against our incurring such indebtedness or engaging in certain reorganizations or transactions. As a result, we could become highly leveraged.

Limitation on Liens under Senior Debt Securities

    In the case of debt securities issued under the senior debt indenture, we have agreed, among other things, to not, directly or indirectly, create, issue, assume, incur or guarantee any indebtedness for money borrowed which is secured by a mortgage, pledge, lien, security interest or other encumbrance of any nature on any of the present or future common stock of any of our significant subsidiaries (or any company besides us having direct or indirect control of any of our significant subsidiaries). This restriction will not apply if we ensure that the debt securities (together with, if we decide, any other indebtedness for money borrowed by us then existing or thereafter created which is not subordinate to the debt securities) will be secured equally and proportionately with (or, at our option, prior to) such other secured indebtedness for as long as such indebtedness is secured. For purposes of this restriction, "significant subsidiary" means any of our subsidiaries that constitutes a "significant subsidiary" as defined in Rule 1-02(w) of Regulation S-X under the Securities Exchange Act of 1934.

Events of Default

    With respect to any series of debt securities, "event of default" means any of the following:

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we fail to pay the interest or any additional amount on any debt security of that series when due and such failure continues for 30 days;

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we fail to pay the principal or any premium on any debt security of that series when due;

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we fail to comply with any of our other agreements contained in the applicable indenture and such failure continues for 90 days after written notice is given to us of that failure from the applicable trustee (or to us and such trustee from the holders of at least 25% in principal amount of the outstanding debt securities of that series);

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certain events of bankruptcy, insolvency or reorganization relating to us; and

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any other event of default provided with respect to debt securities of that series that is described in the applicable prospectus supplement accompanying this prospectus. (Section 6.01)

    If there is a continuing event of default with respect to any outstanding series of debt securities, the applicable trustee or the holders of at least 25% of the outstanding principal amount of the debt securities of that series may require us to pay immediately the principal (or, if the debt securities of that series are discount securities, that portion of the principal amount as may be specified in the terms of that series) of and accrued and unpaid interest, if any, on all debt securities of that series. However, at any time after such trustee or the holders, as the case may be, declare such acceleration with respect to debt securities of any series, but before the applicable person has obtained a judgment or decree for payment of the money, the holders of a majority in principal amount of the outstanding debt securities of that series may, under certain conditions, cancel such acceleration if we have cured all events of default (other than the non-payment of accelerated principal) with respect to debt securities of that series or all such events of default have been waived as provided in the applicable indenture.

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(Section 6.01) For information as to waiver of defaults, see "—Modification and Waiver." We refer you to the prospectus supplement relating to any series of debt securities that are discount securities for the particular provisions relating to acceleration of the maturity of a portion of the principal amount of such debt securities triggered by an event of default.

    Each indenture provides that, subject to the duties of the trustee to act with the required standard of care if there is a continuing event of default, the trustee need not exercise any of its rights or powers under the indenture at the request or direction of any of the holders of debt securities, unless such holders have offered to the trustee reasonable security or indemnity. (Section 6.04) Subject to such provisions for security or indemnification of each trustee and certain other conditions, the holders of the majority in principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the applicable trustee or exercising any trust or power such trustee holds with respect to the debt securities of that series. (Section 6.06)

    No holder of any debt security of any series will have any right to institute any proceeding with respect to either indenture or for any remedy under the applicable indenture unless:

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the applicable trustee has failed to institute such proceeding for 60 days after the holder has previously given to such trustee written notice of a continuing event of default with respect to debt securities of that series;

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the holders of at least 25% in principal amount of the outstanding debt securities of that series have made written request, and offered reasonable security or indemnity, to the applicable trustee to institute such proceeding as trustee; and

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the applicable trustee has not received from the holders of a majority in principal amount of the outstanding debt securities of that series a direction inconsistent with such request. (Section 6.04)

However, the holder of any debt security will have an absolute and unconditional right to receive payment of the principal of, and any premium or interest on, and any additional amounts with respect to, such debt security on or after the date or dates they are to be paid as expressed in such debt security and to institute suit for the enforcement of any such payment. (Section 6.04)

    We are required to furnish to each trustee annually a statement as to the existence or absence of certain defaults under each indenture. (Section 4.06) Each indenture provides that the trustee shall provide notice to the holders of debt securities of any series within 90 days of the occurrence of any default with respect to debt securities of that series known to the trustee, except that the trustee need not provide holders of debt securities of any series notice of any default (other than the non-payment of principal or any premium, interest or additional amounts) if it considers it in the interest of the holders of debt securities of that series not to provide such notice. (Section 6.07)

Consolidation and Merger

    Each indenture provides that we may consolidate with or merge into, or transfer or lease our assets substantially as an entirety to, another person without the consent of any debt security holders if, along with certain other conditions in the indentures:

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the person (if other than us) formed by such consolidation or into which we merge or which acquires or leases our assets is a corporation organized and existing under the laws of the United States, any state thereof or the District of Columbia and expressly assumes our obligations on the debt securities and under the applicable indenture; and

•

after giving effect to such transaction, there is no event of default, and no event which, after notice or passage of time or both, would become an event of default. (Section 11.01)

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Defeasance

    Defeasance and Discharge.  Unless the debt securities of any series provide otherwise, we may be discharged from any and all obligations in respect of the debt securities of that series (except for certain obligations to register the transfer or exchange of debt securities of that series, to replace stolen, destroyed, lost or mutilated debt securities of that series, to maintain paying agencies, to compensate and indemnify the applicable trustee or to furnish such trustee (if the trustee is not the registrar) with the names and addresses of holders of debt securities of that series). This discharge, referred to as defeasance, will occur only if, among other things:

•

we irrevocably deposit with the applicable trustee, in trust, money and/or securities of the government which issues the currency in which the debt securities of that series are payable or securities of agencies backed by the full faith and credit of such government, which, through the payment of interest and principal in accordance with their terms, will provide enough money to pay each installment of principal of, and any premium and interest on, and any additional amounts and any mandatory sinking fund payments in respect of, the debt securities of that series on the applicable due dates for those payments in accordance with the terms of those debt securities; and

•

we deliver to the applicable trustee an opinion of counsel confirming that the holders of the debt securities of that series will not recognize income, gain or loss for United States federal income tax purposes as a result of such defeasance and will be subject to United States federal income tax on the same amounts and in the same manner and at the same times as would have been the case if the discharge had not occurred.

That opinion must state that we have received from, or there has been published by, the United States Internal Revenue Service a ruling or, since the date of execution of the applicable indenture, there has been a change in the applicable United States federal income tax law, in any case, in support of that opinion. (Sections 13.02 and 13.04)

    In addition, we may also obtain a discharge of either indenture with respect to all debt securities issued under such indenture by depositing with the applicable trustee, in trust, enough money to pay all amounts due on the debt securities on the date such payments are due or upon redemption of all of such debt securities, so long as such debt securities are by their terms to become due and payable within one year or are to be called for redemption within one year. (Section 12.01)

    Defeasance of Certain Covenants and Certain Events of Default.  Unless the debt securities of any series provide otherwise, upon compliance with certain conditions:

•

we may omit to comply with any provision of the applicable indenture (except for certain obligations to register the transfer or exchange of debt securities of that series, to replace stolen, destroyed, lost or mutilated debt securities of that series, to maintain paying agencies, to compensate and indemnify the applicable trustee or to furnish such trustee (if the trustee is not the registrar) with the names and addresses of holders of debt securities of that series), including the covenants described under "—Limitation on Liens under Senior Debt Securities" (with respect to senior debt securities) and "—Consolidation and Merger"; and

•

any omission to comply with those covenants will not constitute an event of default with respect to the debt securities of that series ("covenant defeasance"). (Sections 13.03 and 13.04)

The conditions include:

•

depositing with the applicable trustee money and/or securities of the government which issues the currency in which the debt securities of that series are payable or securities of agencies backed by the full faith and credit of such government, which, through the payment of interest and principal in accordance with their terms, will provide enough money to pay each installment of principal of, any premium and interest on, and any additional amounts and any mandatory

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sinking fund payments in respect of, the debt securities of that series on the due dates for those payments in accordance with the terms of those debt securities; and

•

delivering to the applicable trustee an opinion of counsel to the effect that the holders of the debt securities of that series will not recognize income, gain or loss for United States federal income tax purposes as a result of the deposit and related covenant defeasance and will be subject to United States federal income tax on the same amounts and in the same manner and at the same times as would have been the case if the deposit and related covenant defeasance had not occurred. (Section 13.04)

    Covenant Defeasance and Certain Other Events of Default.  If we exercise our option to effect a covenant defeasance with respect to the debt securities of any series as described above and the debt securities of that series are thereafter declared due and payable because of an event of default (other than an event of default caused by failing to comply with the covenants that are defeased), the amount of money and securities we have deposited with the applicable trustee would be sufficient to pay amounts due on the debt securities of that series on their respective due dates but may not be sufficient to pay amounts due on the debt securities of that series at the time of acceleration resulting from such event of default. However, we would remain liable for any shortfall.

Modification and Waiver

    Each indenture provides that we may enter into supplemental indentures with the trustee without the consent of the holders of debt securities to:

•

document the fact that a successor corporation has assumed our obligations;

•

add covenants or events of default for the protection of the holders of debt securities;

•

add or change provisions as are necessary to permit issuance of global debt securities or unregistered securities and to facilitate the exchangeability of such debt securities with registered securities and the issuance of uncertificated debt securities of any series;

•

cure any ambiguity or correct any inconsistency in the indenture;

•

document the fact that a successor trustee has been appointed; or

•

establish the forms and terms of debt securities of any series. (Section 10.01)

    In addition, the subordinated debt indenture provides that we may enter into a supplemental indenture with the trustee without the consent of the holders of subordinated debt securities to provide for the terms and conditions of conversion into common stock if such terms and conditions are different than those provided in the subordinated debt indenture. (Section 10.01 of the subordinated debt indenture)

    We may modify either indenture with the consent of the trustee and holders of at least a majority in principal amount of outstanding debt securities of each series affected by such modification. However, we may not modify either indenture without the consent of the holders of all then outstanding debt securities of the affected series to:

•

change the due date of the principal of, or any installment of principal of or interest on, or payment of additional amounts with respect to, the debt securities of that series;

•

reduce the principal amount of, or any premium or interest rate on, or any additional amounts with respect to, the debt securities of that series;

•

reduce the amount due and payable upon acceleration or make payments thereon payable in any currency other than that provided in such debt security;

•

impair the right to institute suit for the enforcement of any such payment on or after it is due; or

10

•

reduce the percentage in principal amount of outstanding debt securities of any series, the consent of whose holders is necessary to effect any such modification or amendment of the indenture, for waiver of compliance with certain covenants and provisions in the indenture or for waiver of certain defaults. (Section 10.02)

In the case of the subordinated debt indenture, no modification may adversely affect the rights of any holder of senior indebtedness under the subordination provisions of the subordinated debt indenture without the consent of such holder. (Section 10.02 of the subordinated debt indenture)

    The holders of a majority in aggregate principal amount of the outstanding debt securities of any series issued under the senior debt indenture may on behalf of the holders of all debt securities of that series waive, insofar as that series is concerned, compliance by us with the restrictive covenant of the senior debt securities described above under "—Limitation on Liens under Senior Debt Securities." The holders of at least a majority in principal amount of the outstanding debt securities of any series may on behalf of the holders of all debt securities of that series waive any past default under either indenture with respect to that series, except a default in the payment of the principal of or any premium or any interest on, any debt security of that series or in respect of a provision which under the indenture cannot be modified or amended without the consent of the holder of each outstanding debt security of that affected series. (Section 6.09)

Global Securities

    The registered debt securities of a series may be issued in the form of one or more fully registered global securities that will be deposited with The Depository Trust Company, New York, New York ("DTC") or its nominee. This means that we will not issue certificates to each holder. Each global security will be issued to DTC, which will keep a computerized record of its participants (for example, your broker) whose clients have purchased debt securities. The participant will then keep a record of its clients who purchased the debt securities. Unless it is exchanged in whole or in part for a certificate, a global security may not be transferred, except that DTC, its nominees, and their successors may transfer a global security as a whole to one another.

    Beneficial interests in global securities will be shown on, and transfers of global securities will be made only through, records maintained by DTC and its participants. If you are not a participant in DTC, you may beneficially own debt securities held by DTC only through a participant.

    The laws of some states require that certain purchasers of securities take physical delivery of such securities in definitive form. Such limits and laws may impair the ability to transfer beneficial interests in a global security.

    DTC has provided us the following information: DTC will act as securities depository for the debt securities represented by one or more global securities. The debt securities will be issued as fully-registered debt securities registered in the name of Cede & Co. (DTC's partnership nominee) or such other name as may be requested by an authorized representative of DTC. One fully-registered global security certificate will be issued for each issue of the debt securities, in the aggregate principal amount of the issue, and will be deposited with DTC. If, however, the aggregate principal amount of any issue exceeds the maximum principal amount permitted by DTC, one global certificate will be issued with respect to the maximum principal amount and additional global certificates will be issued with respect to any remaining principal amount of that issue.

    DTC is a limited-purpose trust company organized under the New York Banking Law, a "banking organization" within the meaning of the New York Banking Law, a member of the Federal Reserve System, a "clearing corporation" within the meaning of the New York Uniform Commercial Code, and a "clearing agency" registered pursuant to the provisions of Section 17A of the Securities Exchange Act of 1934.

    DTC holds securities that its direct participants deposit with DTC. DTC also facilitates the settlement among direct participants of securities transactions, such as transfers and pledges, in

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deposited securities through electronic computerized book-entry changes in direct participants' accounts, thereby eliminating the need for physical movement of securities certificates. Direct participants include securities brokers and dealers, banks, trust companies, clearing corporations, and certain other organizations.

    DTC is owned by a number of its direct participants and by the New York Stock Exchange, Inc., the American Stock Exchange LLC, and the National Association of Securities Dealers, Inc. Access to the DTC system is also available to others such as securities brokers and dealers, banks, and trust companies that clear through or maintain a custodial relationship with a direct participant, either directly or indirectly. The rules applicable to DTC and its participants are on file with the SEC.

    Purchases of debt securities represented by one or more global securities under the DTC system must be made by or through direct participants, which will receive a credit for the debt securities on DTC's records. The ownership interest of each beneficial owner of each debt security is in turn to be recorded on the direct and indirect participants' records. Beneficial owners will not receive written confirmation from DTC of their purchase, but beneficial owners are expected to receive written confirmations providing details of the transaction, as well as periodic statements of their holdings, from the direct or indirect participant through which the beneficial owner entered into the transaction. Transfers of ownership interests in the debt securities are to be accomplished by entries made on the books of participants acting on behalf of beneficial owners. Beneficial owners will not receive certificates representing their ownership interests in debt securities, except in the event that use of the book-entry system for the debt securities is discontinued.

    To facilitate subsequent transfers, all debt securities deposited by direct participants with DTC are registered in the name of DTC's partnership nominee, Cede & Co., or such other name as may be requested by an authorized representative of DTC. The deposit of debt securities with DTC and their registration in the name of Cede & Co. or such other nominee do not effect any change in beneficial ownership. DTC has no knowledge of the actual beneficial owners of the debt securities; DTC's records reflect only the identity of the direct participants to whose accounts such debt securities are credited, which may or may not be the beneficial owners. The participants will remain responsible for keeping account of their holdings on behalf of their customers.

    Delivery of notices and other communications by DTC to direct participants, by direct participants to indirect participants, and by direct participants and indirect participants to beneficial owners will be governed by arrangements among them, subject to any statutory or regulatory requirements as may be in effect from time to time. Redemption notices, if any, will be sent to DTC. If less than all of the debt securities within an issue are being redeemed, DTC's practice is to determine by lot the amount of the interest of each direct participant in such issue to be redeemed.

    Neither DTC nor Cede & Co. (nor such other DTC nominee) will consent or vote with respect to the debt securities. Under its usual procedures, DTC mails an omnibus proxy to us as soon as possible after the record date. The omnibus proxy assigns Cede & Co.'s consenting or voting rights to those direct participants to whose accounts the debt securities are credited on the record date (identified in a listing attached to the omnibus proxy).

    Redemption proceeds, distributions, and dividend payments on the debt securities will be made to Cede & Co., or such other nominee as may be requested by an authorized representative of DTC. DTC's practice is to credit direct participants' accounts, upon DTC's receipt of funds and corresponding detail information from us or the paying agent on payable date in accordance with their respective holdings shown on DTC's records. Payments by participants to beneficial owners will be governed by standing instructions and customary practices, as is the case with securities held for the accounts of customers in bearer form or registered in "street name," and will be the responsibility of each participant and not of DTC, the paying agent, or us, subject to any statutory or regulatory requirements as may be in effect from time to time. Payment of redemption proceeds, distributions, and dividends to Cede & Co. (or such other nominee as may be requested by an authorized

12

representative of DTC) is the responsibility of us or the paying agent, disbursement of such payments to direct participants will be the responsibility of DTC, and disbursement of such payments to the beneficial owners will be the responsibility of direct and indirect participants.

    A beneficial owner must give notice to elect to have its debt securities purchased or tendered, through its participant, to the paying agent, and will effect delivery of the debt securities by causing the direct participant to transfer the participant's interest in the debt securities, on DTC's records, to the paying agent. The requirement for physical delivery of the debt securities in connection with an optional tender or a mandatory purchase will be deemed satisfied when the ownership rights in the debt securities are transferred by direct participants on DTC's records and followed by a book-entry credit of tendered securities to the paying agent's DTC account.

    DTC may discontinue providing its services as securities depository with respect to the debt securities at any time by giving reasonable notice to us or the paying agent. Under such circumstances, in the event that a successor securities depository is not obtained, debt security certificates are required to be printed and delivered. We may decide to discontinue use of the system of book-entry transfers through DTC (or a successor securities depository). In that event, debt security certificates will be printed and delivered.

    The information in this section concerning DTC and DTC's book-entry system has been obtained from sources that we believe to be reliable, but we takes no responsibility for its accuracy.

    The debt securities of a series may also be issued in the form of one or more bearer global securities that will be deposited with a common depositary for Euroclear Bank S.A. / N.V., as operator of the Euroclear System and Clearstream Banking, société anonyme, or with a nominee for such depositary identified in the applicable prospectus supplement relating to such series. The applicable prospectus supplement will describe the specific terms and procedures, including the specific terms of the depositary arrangement, with respect to any portion of a series of debt securities to be represented by a bearer global security.

    Neither we nor any underwriter or agent, applicable trustee, paying agent or registrar of any debt securities will have any responsibility or liability for any aspect of the records relating to or payments made on account of beneficial ownership interests in a global security, or for maintaining, supervising or reviewing any records relating to such beneficial ownership interests.

Subordination under the Subordinated Debt Indenture

    The subordinated debt securities issued under the subordinated debt indenture will be subordinate and junior in right of payment to all senior indebtedness to the extent provided in the subordinated debt indenture. (Section 14.03 of the subordinated debt indenture) We may not make any payments on account of principal or any premium, redemption, interest or other amount on the subordinated debt securities at any time when we have defaulted with respect to payment of principal or any premium, interest, sinking fund or other payment due on the senior indebtedness. (Section 14.02 of the subordinated debt indenture) If we make any payment described in the foregoing sentence under the subordinated debt indenture before all senior indebtedness is paid in full, such payment or distribution will be applied to pay off the senior indebtedness which remains unpaid. Subject to the condition that the senior indebtedness is paid in full, if any such payments are made on the senior indebtedness as described above, the subordinated debt security holders will be subrogated to the rights of the senior debt security holders. (Section 14.03 of the subordinated debt indenture)

    The subordinated debt indenture defines the term "senior indebtedness" to mean:

•

all indebtedness of Aon Corporation, whether outstanding on the date of the subordinated debt indenture or created later, for money borrowed or otherwise evidenced by a note or similar instrument given in connection with the acquisition of any business, property or assets (other than inventory or other similar property acquired in the ordinary course of business), including

13

securities or for the payment of money relating to a capitalized lease obligation (as defined in the subordinated debt indenture);

•

any indebtedness of others described in the preceding bullet point which we have guaranteed or which is otherwise our legal obligation;

•

any of our indebtedness under interest rate swaps, caps or similar hedging agreements and foreign exchange contracts, currency swaps or similar agreements; and

•

renewals, extensions, refundings, restructurings, amendments and modifications of any indebtedness or guarantee described above. (Section 1.01 of the subordinated debt indenture)

    "Senior indebtedness" does not include:

•

any of our indebtedness to any of our subsidiaries; or

•

any of our indebtedness which by its terms is equal or subordinated to the subordinated debt securities in rights of payment or upon liquidation. (Section 1.01 of the subordinated debt indenture)

    Because of the subordination provisions described above, some of our general creditors may recover proportionately more than holders of the subordinated debt securities if our assets are distributed as a result of insolvency or bankruptcy. The subordinated debt indenture provides that the subordination provisions will not apply to money and securities held in trust pursuant to the satisfaction and discharge and the legal defeasance provisions of the subordinated debt indenture. (Section 14.03 of the subordinated debt indenture) See "—Defeasance" for additional information regarding the legal defeasance provisions affecting the subordinated debt.

    We will set forth (or incorporate by reference) the approximate amount of senior indebtedness outstanding as of a recent date in any prospectus supplement under which we offer to sell subordinated debt securities.

Conversion Rights

    We will include in a supplement to this prospectus the terms and conditions, if any, on which debt securities being offered are convertible into common stock. Such terms will include the conversion price, the conversion period, provisions as to whether conversion will be at our option or the option of the holder, the events requiring an adjustment of the conversion price and provisions affecting conversion in the event of the redemption of such debt securities.

Regarding the Trustees

    We have commercial deposits and custodial arrangements with the senior indenture trustee and have borrowed money from such trustee in the normal course of business. We also have commercial deposits with the subordinated indenture trustee. We may enter into similar or other banking relationships with either of the trustees in the future in the normal course of business. In addition, we have provided brokerage and other insurance services in the ordinary course of their respective businesses for each trustee. The senior indenture trustee is also trustee with respect to other debt securities we have issued.

DESCRIPTION OF PREFERRED STOCK AND COMMON STOCK

    Our second restated certificate of incorporation, as amended, authorizes us to issue 750,000,000 shares of common stock, par value $1.00 per share, and 25,000,000 shares of serial preferred stock, par value $1.00 per share. In general, any series of preferred stock is afforded preferences regarding dividends and liquidation rights over the common stock. The second restated certificate of incorporation, as amended, empowers the board of directors of Aon Corporation, without approval of the stockholders, to cause preferred stock to be issued in one or more series, with the numbers of shares of each series and the rights, preferences and limitations of each series to be determined by it.

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The description set forth below is only a summary and is not complete. For more information regarding the preferred stock and common stock which may be offered by this prospectus, please refer to the applicable prospectus supplement and our second restated certificate of incorporation, as amended, which is incorporated by reference as an exhibit to the registration statement. In addition, a more detailed description of the common stock may be found in the documents referred to in the fourth bullet point in the second paragraph of "Where You Find More Information."

    Because we are a holding company, the holders of the preferred and common stock may not receive assets of our subsidiaries in a liquidation or recapitalization of Aon Corporation and its subsidiaries until the claims of the subsidiaries' creditors and insurance policyholders (in the case of insurance subsidiaries) are paid, except to the extent that we may have recognized claims against such subsidiaries. In addition, certain regulatory laws limit some of our subsidiaries from making payments to us of dividends and on loans and other transfers of funds.

Preferred Stock

    We will include in a supplement to this prospectus the terms relating to any preferred stock being offered. These terms will include some or all of the following:

•

the number of shares;

•

the designation of the series;

•

the initial offering price;

•

any liquidation preference per share;

•

any dividend rights and the specific terms relating thereto;

•

whether and upon what terms the shares will be redeemable;

•

whether and upon what terms the shares will have a sinking fund to be used to purchase or redeem the shares of any series;

•

whether and upon what terms the shares will be convertible or exchangeable into another security;

•

the relative priority of such shares to other classes or series of preferred stock with respect to rights and preferences;

•

any provisions for the auction or remarketing of the preferred stock;

•

the restrictions, if any, on the issue or reissue of any additional preferred stock, including increases or decreases in the number of shares of any series subsequent to the issue of shares of that series;

•

any voting rights;

•

whether or not the shares are or will be listed on any securities exchange;

•

any additional terms, preferences, rights, limitations or restrictions applicable to the shares; and

•

a discussion of Federal income tax considerations applicable to the shares.

Common Stock

    We will include in a supplement to this prospectus the terms of any offering of our common stock, including the number of shares offered, the initial offering price, market price and dividend information.

    Common stockholders will receive dividends as may be declared at various times by the board of directors out of funds legally available for that purpose. Common stockholders are entitled to one vote per share on all matters submitted to a vote of stockholders and do not have cumulative voting rights. Common stockholders will receive, upon any liquidation of Aon Corporation, all remaining assets

15

available for distribution to stockholders after we satisfy our liabilities relating to, and make payments in respect of preferential obligations of, any preferred stock that may then be issued and outstanding. Common stockholders have no preemptive rights. The common stock is listed on the New York, Chicago, London and Frankfurt Stock Exchanges. Equiserve Trust Company, N. A. is the registrar and transfer agent for the common stock.

Certain Anti-Takeover Provisions

    Our second restated certificate of incorporation, as amended, contains provisions, summarized below, that could have the effect of delaying, deferring or preventing a change of control of Aon Corporation. Because this is a summary, it does not contain all of the information that may be important to you. You should read carefully the provisions of our second restated certificate of incorporation, as amended, as well as the provisions of any applicable laws.

    Our second restated certificate of incorporation, as amended, provides that the approval of a voluntary liquidation or dissolution of Aon Corporation and certain business combinations (including mergers, consolidations, sales, leases and exchanges), requires the affirmative vote of at least two-thirds of all of the securities of Aon Corporation then entitled to vote at a meeting of stockholders, considered as one class. Our second restated certificate of incorporation, as amended, also permits our board of directors, in response to certain acquisition proposals (including tender or exchange offers, mergers, consolidations and sales), to consider not only the best interests of the stockholders, but also such other factors as the board of directors deems relevant, including social, legal and economic effects upon employees, field sales agents, suppliers, customers, policyholders and business. In addition, unless the board of directors decides otherwise with respect to any series of preferred stock, stockholders may not take any action by written consent if such action is the type that must or may be taken at any annual or special meeting of stockholders.

    Under Section 203 of the Delaware General Corporation Law, we may not engage in certain business combinations (as defined in such section) with any interested stockholders (as defined in such section) for a period of three years following the date that such stockholder became an interested stockholder, unless:

•

prior to such date our board of directors approved the business combination with the interested stockholder;

•

the interested stockholder owned at least 85% of our voting stock upon consummation of the transaction which resulted in the stockholder becoming an interested stockholder; or

•

the business combination is approved by the affirmative vote of at least two-thirds of the outstanding voting stock.

    Our board of directors has adopted a resolution making the provisions of Section 203 inapplicable to transactions involving Mr. Patrick G. Ryan, our Chairman and Chief Executive Officer. In addition, the insurance laws and regulations of the jurisdictions in which our subsidiaries do business may impede or delay a business combination involving Aon Corporation.

DESCRIPTION OF THE SHARE PURCHASE CONTRACTS
AND THE SHARE PURCHASE UNITS

    We may issue share purchase contracts, representing contracts obligating holders to purchase from us, and obligating us to sell to the holders, a specified number of shares of our common stock at a future date or dates. The price per share and the number of shares of our common stock may be fixed at the time the share purchase contracts are issued or may be determined by reference to a specific formula set forth in the share purchase contracts. The share purchase contracts may be issued separately or as a part of share purchase units consisting of a share purchase contract and, as security for the holder's obligations to purchase the shares under the share purchase contracts, either:

•

senior debt securities or subordinated debt securities;

16

•

shares of preferred stock; or

•

debt obligations of third parties, including U.S. Treasury securities.

    The share purchase contracts may require us to make periodic payments to the holders of the share purchase units or vice versa, and such payments may be unsecured or prefunded on some basis. The share purchase contracts may require holders to secure their obligations in a specified manner and, in certain circumstances, we may deliver newly issued prepaid share purchase contracts upon release to a holder of any collateral securing such holder's obligations under the original share purchase contract.

    The applicable prospectus supplement will describe the terms of any share purchase contracts or share purchase units and, if applicable, prepaid share purchase contracts. The description in the prospectus supplement will not purport to be complete and will be qualified in its entirety by reference to:

•

the share purchase contracts;

•

the collateral arrangements and depositary arrangements, if applicable, relating to such share purchase contracts or share purchase units; and

•

if applicable, the prepaid share purchase contracts and the document pursuant to which such prepaid share purchase contracts will be issued.

PLAN OF DISTRIBUTION

    We may sell the securities in any of the following ways:

•

directly to purchasers;

•

through agents;

•

through dealers; or

•

through one or more underwriters or a syndicate of underwriters in an underwritten offering.

    The applicable prospectus supplement will set forth the names of any underwriters or agents involved in the sale of the securities being offered and any applicable commissions or discounts.

    Underwriters, dealers or agents may offer and sell the securities at a fixed price or prices, which may be changed, or from time to time at market prices prevailing at the time of sale, at prices related to such prevailing market prices or at negotiated prices. In connection with the sale of the securities, underwriters or agents may be deemed to have received compensation from us in the form of underwriting discounts or commissions and may also receive commissions from purchasers of the securities for whom they may act as agent. Underwriters or agents may sell the securities to or through dealers, and such dealers may receive compensation in the form of discounts, concessions or commissions from the underwriters or commissions from the purchasers for whom they may act as agent.

    If we do not list the securities being offered on a national securities exchange, any underwriters or agents to or through whom such securities are sold by us for public offering and sale may make a market in such securities, but such underwriters or agents will not be obligated to do so and may discontinue any market making at any time without notice. We give no assurances as to the liquidity of the trading market for any of the securities being offered.

    Any underwriters, dealers or agents participating in the distribution of the securities may be deemed to be underwriters and any discounts and commissions received by them and any profit realized by them on resale of the securities may be deemed to be underwriting discounts and commissions under the Securities Act of 1933. We may enter into agreements with underwriters, dealers or agents under which we agree to indemnify against, or contribute payments made in respect of, certain civil liabilities incurred by such persons, including liabilities under the Securities Act of 1933.

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    If so indicated in the applicable prospectus supplement, we will authorize underwriters or agents to solicit offers by certain institutions to purchase securities from us pursuant to contracts providing for payment and delivery on a future date. These contracts may be made with commercial and savings banks, insurance companies, pension funds, investment companies, educational and charitable institutions and others, but in all cases such institutions must be approved by us. The obligations of any purchaser under any such contract will be subject to the condition that the purchase of the securities is at the time of delivery not prohibited under the laws of the jurisdiction to which such purchaser is subject. The underwriters and such other persons will not have any responsibility in respect of the validity or performance of such contracts.

VALIDITY OF SECURITIES

    The validity of the securities will be passed upon for us by Richard E. Barry, our Senior Counsel and Assistant Secretary. As of November 30, 2001, Mr. Barry owned 4,059 shares of Aon Corporation common stock, held restricted stock awards of 37,345 shares and 16,000 unvested stock options. In addition, 3,870 shares of Aon Corporation common stock held by its employee stock ownership plan and savings plan were attributable to Mr. Barry.

EXPERTS

    The consolidated financial statements of Aon Corporation incorporated by reference in Aon Corporation's Annual Report (Form 10-K) for the year ended December 31, 2000, and the related financial statement schedules included therein, have been audited by Ernst & Young LLP, independent auditors, as set forth in their report on the consolidated financial statements incorporated by reference therein and their report on the related financial statement schedules included therein, both incorporated herein by reference. Such consolidated financial statements and schedules are incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.

    With respect to the unaudited condensed consolidated interim financial information for the three-month periods ended March 31, 2001 and March 31, 2000, the three and six-month periods ended June 30, 2001 and June 30, 2000, and the three and nine-month periods ended September 30, 2001 and September 30, 2000, incorporated by reference in this prospectus, Ernst & Young LLP have reported that they have applied limited procedures in accordance with professional standards for a review of such information. However, their separate reports, included in Aon Corporation's Quarterly Reports on Form 10-Q for the quarters ended March 31, 2001, June 30, 2001, and September 30, 2001, and incorporated herein by reference, state they did not audit and they do not express an opinion on that interim financial information. Accordingly, the degree of reliance on their reports on such information should be restricted considering the limited nature of the review procedures applied. The independent auditors are not subject to the liability provisions of Section 11 of the Securities Act of 1933 (the "Act") for their report on the unaudited interim financial information because that report is not a "report" or a "part" of the registration statement prepared or certified by the auditors within the meaning of Sections 7 and 11 of the Act.

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PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

Item 14. Other Expenses of Issuance and Distribution.*

Securities and Exchange Commission registration fee		$	179,250
Accounting fees			50,000
Trustee's fees and expenses			5,000
Printing, distribution, and engraving fees			50,000
Rating agency fees			200,000
Legal fees and expenses			100,000
State qualification fees and expenses			5,000
Miscellaneous			10,750
	TOTAL	$	600,000

    *All amounts are estimated except for the Securities and Exchange Commission registration fee.

Item 15. Indemnification of Directors and Officers.

    The registrant was organized under and is subject to the Delaware General Corporation Law. Delaware law provides that officers and directors may receive indemnification from their corporations for certain actual or threatened lawsuits. The Delaware law sets out the standard of conduct which the officers and directors must meet in order to be indemnified, the parties who are to determine whether the standard has been met, and the types of expenditures which will be indemnified. Delaware law further provides that a corporation may purchase indemnification insurance, such insurance providing indemnification for the officers and directors whether or not the corporation would have the power to indemnify them against such liability under the provisions of Delaware law.

    The registrant has adopted an article within its second restated certificate of incorporation, as amended, which provides that it will indemnify its officers and directors to the full extent permitted by Delaware law.

    Furthermore, the registrant is covered by insurance which will reimburse it within the policy limits for amounts it is obligated to pay in lawsuits involving officers and directors serving in such capacities in which the damages, judgments, settlements, costs, charges or expenses incurred in connection with the defense of the action, suit or proceeding are reimbursable pursuant to the law and the second restated certificate of incorporation, as amended.

    The registrant expects that any underwriting agreement or distribution agreement relating to the securities will provide for indemnification of directors and officers of the registrant by the underwriters or agents, as the case may be, against certain liabilities.

II–1

Item 16. Exhibits.

    Exhibits marked with an asterisk (*) are incorporated by reference to documents previously filed by the registrant with the Securities and Exchange Commission, as indicated. All other documents listed are or will be filed with this registration statement.

Number	Description
*1(a)	Form of Underwriting Agreement—incorporated by reference to Exhibit 1(a) of the registrant's Registration Statement on Form S-3 No. 333-78723 dated May 18, 1999.
*3(a)	Second Restated Certificate of Incorporation of the registrant—incorporated by reference to Exhibit 3(a) to the registrant's Annual Report on Form 10-K for the year ended December 31, 1991, file no. 1-7933.
*3(b)	Certificate of Amendment of the registrant's Second Restated Certificate of Incorporation—incorporated by reference to Exhibit 3 to the registrant's Quarterly Report on Form 10-Q for the quarter ended March 31, 1994, file no. 1-7933.
*3(c)	Certificate of Amendment of the registrant's Second Restated Certificate of Incorporation—incorporated by reference to Exhibit 3 to the registrant's Current Report on Form 8-K filed on May 9, 2000, file no. 1-7933.
*3(d)	Amended by-laws of the registrant incorporated by reference to Exhibit 3(d) to the registrant's Annual Report on Form 10-K for the year ended December 31, 2000, file no. 1-7933.
*3(e)	By-laws of the registrant—incorporated by reference to Exhibit 3(d) to the registrant's Annual Report on Form 10-K for the year ended December 31, 1982, file no. 1-7933.
*4(a)	Certificate of Designation for the registrant's Series C Cumulative Preferred Stock—incorporated by reference to Exhibit 4.1 to the registrant's Current Report on Form 8-K dated February 9, 1994, file no. 1-7944.
*4(b)	Indenture dated September 15, 1992 between the registrant and The Bank of New York (successor to Continental Bank, National Association which is now known as Bank of America National Trust and Savings Association), as trustee, relating to the registrant's senior debt securities—incorporated by reference to Exhibit 4(a) to the registrant's Current Report on Form 8-K dated September 23, 1992, file no. 1-7933.
*4(c)	Form of senior debt securities—incorporated by reference to Exhibit 4(b) to the registrant's Current Report on Form 8-K dated September 23, 1992, file no. 1-7933.
*4(d)	Resolutions establishing terms of 7.40% Notes Due 2002—incorporated by reference to Exhibit 4(d) to the registrant's Annual Report on Form 10-K for the year ended December 31, 1992, file no. 1-7933.
*4(e)	Resolutions establishing the terms of 6.70% Notes Due 2003 and 6.30% Notes Due 2004—incorporated by reference to Exhibits 4(c) and 4(d) of the registrant's Annual Report on Form 10-K for the year ended December 31, 1993, file no. 1-7933.
*4(f)	Resolutions establishing the terms of 6.90% Notes Due 2004—incorporated by reference to Exhibit 4(e) of the registrant's Annual Report on Form 10-K for the year ended December 31, 1999, file no. 1-7933.
*4(g)	Form of Indenture relating to the Company's subordinated debt securities—incorporated by reference to Exhibit 4(f) of the registrant's Registration Statement on Form S-3 No. 333-78723 dated May 18, 1999.
4(h)	Form of subordinated debt securities. The form or forms of the subordinated debt securities with respect to each particular offering will be filed as an exhibit subsequently included or incorporated by reference herein.

II–2

*4(i)	Junior Subordinated Indenture dated as of January 13, 1997 between the registrant and The Bank of New York, as trustee—incorporated by reference to Exhibit 4.1 of the registrant's Amendment No. 1 to Registration Statement on Form S-4 No. 333-21237 dated March 27, 1997 (the "Capital Securities Registration").
*4(j)	First Supplemental Indenture dated as of January 13, 1997 between the registrant and the Bank of New York, as trustee—incorporated by reference to Exhibit 4.2 of the Capital Securities Registration.
*4(k)	Certificates of Trust of Aon Capital A—incorporated by reference to Exhibit 4.3 of the Capital Securities Registration.
*4(l)	Amended and Restated Trust Agreement of Aon Capital A dated as of January 13, 1997 among the registrant, as depositor, The Bank of New York, as property trustee, The Bank of New York (Delaware), as Delaware trustee, the administrative trustees named therein and the holders, from time to time, of the capital securities—incorporated by reference to Exhibit 4.5 of the Capital Securities Registration.
*4(m)	Capital Securities Guarantee Agreement dated as of January 13, 1997 between the registrant and The Bank of New York, as guarantee trustee—incorporated by reference to Exhibit 4.8 of the Capital Securities Registration.
*4(n)	Capital Securities Exchange and Registration Rights Agreement dated as of January 13, 1997 among the registrant, Aon Capital A and Morgan Stanley & Co. Incorporated and Goldman, Sachs & Co.—incorporated by reference to Exhibit 4.10 of the Capital Securities Registration.
*4(o)	Debenture Exchange and Registration Rights Agreement dated as of January 13, 1997 among the registrant, Aon Capital A and Morgan Stanley & Co. Incorporated and Goldman, Sachs & Co.—incorporated by reference to Exhibit 4.11 of the Capital Securities Registration.
*4(p)	Guarantee Exchange and Registration Rights Agreement dated as of January 13, 1997 among the registrant, Aon Capital A and Morgan Stanley & Co. Incorporated and Goldman, Sachs & Co.—incorporated by reference to Exhibit 4.12 of the Capital Securities Registration.
5	Opinion of Richard E. Barry, Senior Counsel and Assistant Secretary of the registrant.
*12(a)	Statement of computation of ratio of earning to fixed charges—incorporated by reference to Exhibit 12(a) to the registrant's Annual Report on Form 10-K for the year ended December 31, 2000 (the "2000 Form 10-K") and to Exhibit 12(a) to the registrant's Quarterly Report on Form 10-Q for the quarter ended September 30, 2001 (the "2001 Third Quarter Form 10-Q"), file no. 1-7944.
*12(b)	Statement of computation of ratio of earnings to combined fixed charges and preferred stock dividends—incorporated by reference to Exhibit 12(b) to the 2000 Form 10-K and to Exhibit 12(b) to the 2001 Third Quarter Form 10-Q, file no. 1-7944.
23(a)	Consent of Richard E. Barry (included in Exhibit 5).
23(b)	Consent of Ernst & Young LLP.
23(c)	Acknowledgment of Ernst & Young LLP.
24	Powers of Attorney.
25(a)	Form T-1 Statement of Eligibility and Qualification under the Trust Indenture Act of 1939 of The Bank of New York relating to the senior debt securities.
25(b)	Form T-1 Statement of Eligibility and Qualification under the Trust Indenture Act of 1939 of U.S. Bank Trust National Association relating to the subordinated debt securities.

II–3

Item 17. Undertakings.

    The registrant hereby undertakes:

    (1) To file, during any period in which offers or sales are being made of the securities registered hereby, a post-effective amendment to this registration statement:

(i)

To include any prospectus required by Section 10(a)(3) of the Securities Act of 1933;



(ii)

To reflect in the prospectus any facts or events arising after the effective date of the registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than a 20 percent change in the maximum aggregate offering price set forth in the "Calculation of Registration Fee" table in the effective registration statement;



(iii)

To include any material information with respect to the plan of distribution not previously disclosed in the registration statement or any material change to such information in the registration statement;

provided, however, that the undertakings set forth in paragraphs (i) and (ii) above do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in periodic reports filed with or furnished to the Commission by the registrant pursuant to Section 13 or Section 15(d) of the Securities Exchange Act of 1934 that are incorporated by reference in the registration statement.

    (2) That, for the purpose of determining any liability under the Securities Act of 1933, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

    (3) To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

    (4) That, for purposes of determining any liability under the Securities Act of 1933, each filing of the registrant's annual report pursuant to Section 13(a) or Section 15(d) of the Securities Exchange Act of 1934 (and, where applicable, each filing of an employee benefit plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of 1934) that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

    Insofar as indemnification for liabilities arising under the Securities Act of 1933 may be permitted to directors, officers and controlling persons of the registrant pursuant to the provisions described under Item 15 above, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Act and will be governed by the final adjudication of such issue.

II–4

SIGNATURES

    Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing this registration statement on Form S-3 and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Chicago, State of Illinois, as of December 3, 2001.

Aon CORPORATION
By:	/s/ PATRICK G. RYAN Patrick G. Ryan Chairman and Chief Executive Officer

    Pursuant to the requirements of the Securities Act of 1933, this registration statement has been signed below by the following persons in the capacities indicated as of December 3, 2001.

Signature	Title
/s/ PATRICK G. RYAN Patrick G. Ryan	Chairman, Chief Executive Officer and Director (Principal Executive Officer)
/s/ HARVEY N. MEDVIN Harvey N. Medvin	Executive Vice President and Chief Financial Officer (Principal Financial and Accounting Officer)
* Franklin A. Cole	Director
* Edgar D. Jannotta	Director
* Lester B. Knight	Director
* Perry J. Lewis	Director
* Andrew J. McKenna	Director
* Robert S. Morrison	Director
* Richard C. Notebaert	Director
* Michael D. O'Halleran	President, Chief Operating Officer and Director

* Donald S. Perkins	Director
* John W. Rogers, Jr.	Director
* George A. Schaefer	Director
/s/ RAYMOND I. SKILLING Raymond I. Skilling	Executive Vice President, Chief Counsel and Director
* Fred L. Turner	Director
* Arnold R. Weber	Director
* Carolyn Y. Woo	Director

*By:

/s/ RAYMOND I. SKILLING Raymond I. Skilling
Attorney-in-Fact

EXHIBIT INDEX
TO REGISTRATION STATEMENT ON FORM S-3

    Aon CORPORATION

Number	Description
*1(a)	Form of Underwriting Agreement—incorporated by reference to Exhibit 1(a) of the registrant's Registration Statement on Form S-3 No. 333-78723 dated May 18, 1999.
*3(a)	Second Restated Certificate of Incorporation of the registrant—incorporated by reference to Exhibit 3(a) to the registrant's Annual Report on Form 10-K for the year ended December 31, 1991, file no. 1-7933.
*3(b)	Certificate of Amendment of the registrant's Second Restated Certificate of Incorporation—incorporated by reference to Exhibit 3 to the registrant's Quarterly Report on Form 10-Q for the quarter ended March 31, 1994, file no. 1-7933.
*3(c)	Certificate of Amendment of the registrant's Second Restated Certificate of Incorporation—incorporated by reference to Exhibit 3 to the registrant's Current Report on Form 8-K filed on May 9, 2000, file no. 1-7933.
*3(d)	Amended by-laws of the registrant incorporated by reference to Exhibit 3(d) to the registrant's Annual Report on Form 10-K for the year ended December 31, 2000, file no. 1-7933.
*3(e)	By-laws of the registrant—incorporated by reference to Exhibit 3(d) to the registrant's Annual Report on Form 10-K for the year ended December 31, 1982, file no. 1-7933.
*4(a)	Certificate of Designation for the registrant's Series C Cumulative Preferred Stock—incorporated by reference to Exhibit 4.1 to the registrant's Current Report on Form 8-K dated February 9, 1994, file no. 1-7944.
*4(b)	Indenture dated September 15, 1992 between the registrant and The Bank of New York (successor to Continental Bank, National Association which is now known as Bank of America National Trust and Savings Association), as trustee, relating to the registrant's senior debt securities—incorporated by reference to Exhibit 4(a) to the registrant's Current Report on Form 8-K dated September 23, 1992, file no. 1-7933.
*4(c)	Form of senior debt securities—incorporated by reference to Exhibit 4(b) to the registrant's Current Report on Form 8-K dated September 23, 1992, file no. 1-7933.
*4(d)	Resolutions establishing terms of 7.40% Notes Due 2002—incorporated by reference to Exhibit 4(d) to the registrant's Annual Report on Form 10-K for the year ended December 31, 1992, file no. 1-7933.
*4(e)	Resolutions establishing the terms of 6.70% Notes Due 2003 and 6.30% Notes Due 2004—incorporated by reference to Exhibits 4(c) and 4(d) of the registrant's Annual Report on Form 10-K for the year ended December 31, 1993, file no. 1-7933.
*4(f)	Resolutions establishing the terms of 6.90% Notes Due 2004—incorporated by reference to Exhibit 4(e) of the registrant's Annual Report on Form 10-K for the year ended December 31, 1999, file no. 1-7933.
*4(g)	Form of Indenture relating to the Company's subordinated debt securities—incorporated by reference to Exhibit 4(f) of the registrant's Registration Statement on Form S-3 No. 333-78723 dated May 18, 1999.
4(h)	Form of subordinated debt securities. The form or forms of the subordinated debt securities with respect to each particular offering will be filed as an exhibit subsequently included or incorporated by reference herein.

*4(i)	Junior Subordinated Indenture dated as of January 13, 1997 between the registrant and The Bank of New York, as trustee—incorporated by reference to Exhibit 4.1 of the registrant's Amendment No. 1 to Registration Statement on Form S-4 No. 333-21237 dated March 27, 1997 (the "Capital Securities Registration").
*4(j)	First Supplemental Indenture dated as of January 13, 1997 between the registrant and the Bank of New York, as trustee—incorporated by reference to Exhibit 4.2 of the Capital Securities Registration.
*4(k)	Certificates of Trust of Aon Capital A—incorporated by reference to Exhibit 4.3 of the Capital Securities Registration.
*4(l)	Amended and Restated Trust Agreement of Aon Capital A dated as of January 13, 1997 among the registrant, as depositor, The Bank of New York, as property trustee, The Bank of New York (Delaware), as Delaware trustee, the administrative trustees named therein and the holders, from time to time, of the capital securities—incorporated by reference to Exhibit 4.5 of the Capital Securities Registration.
*4(m)	Capital Securities Guarantee Agreement dated as of January 13, 1997 between the registrant and The Bank of New York, as guarantee trustee—incorporated by reference to Exhibit 4.8 of the Capital Securities Registration.
*4(n)	Capital Securities Exchange and Registration Rights Agreement dated as of January 13, 1997 among the registrant, Aon Capital A and Morgan Stanley & Co. Incorporated and Goldman, Sachs & Co.—incorporated by reference to Exhibit 4.10 of the Capital Securities Registration.
*4(o)	Debenture Exchange and Registration Rights Agreement dated as of January 13, 1997 among the registrant, Aon Capital A and Morgan Stanley & Co. Incorporated and Goldman, Sachs & Co.—incorporated by reference to Exhibit 4.11 of the Capital Securities Registration.
*4(p)	Guarantee Exchange and Registration Rights Agreement dated as of January 13, 1997 among the registrant, Aon Capital A and Morgan Stanley & Co. Incorporated and Goldman, Sachs & Co.—incorporated by reference to Exhibit 4.12 of the Capital Securities Registration.
5	Opinion of Richard E. Barry, Senior Counsel and Assistant Secretary of the registrant.
*12(a)	Statement of computation of ratio of earning to fixed charges—incorporated by reference to Exhibit 12(a) to the registrant's Annual Report on Form 10-K for the year ended December 31, 2000 (the "2000 Form 10-K") and to Exhibit 12(a) to the registrant's Quarterly Report on Form 10-Q for the quarter ended September 30, 2001 (the "2001 Third Quarter Form 10-Q"), file no. 1-7944.
*12(b)	Statement of computation of ratio of earnings to combined fixed charges and preferred stock dividends—incorporated by reference to Exhibit 12(b) to the 2000 Form 10-K and to Exhibit 12(b) to the 2001 Third Quarter Form 10-Q, file no. 1-7944.
23(a)	Consent of Richard E. Barry (included in Exhibit 5).
23(b)	Consent of Ernst & Young LLP.
23(c)	Acknowledgment of Ernst & Young LLP.
24	Powers of Attorney.
25(a)	Form T-1 Statement of Eligibility and Qualification under the Trust Indenture Act of 1939 of The Bank of New York relating to the senior debt securities.
25(b)	Form T-1 Statement of Eligibility and Qualification under the Trust Indenture Act of 1939 of U.S. Bank Trust National Association relating to the subordinated debt securities.

QuickLinks

Aon Corporation
ABOUT THIS PROSPECTUS
WHERE YOU CAN FIND MORE INFORMATION
THE COMPANY
USE OF PROCEEDS
RATIOS
DESCRIPTION OF DEBT SECURITIES
DESCRIPTION OF PREFERRED STOCK AND COMMON STOCK
DESCRIPTION OF THE SHARE PURCHASE CONTRACTS AND THE SHARE PURCHASE UNITS
PLAN OF DISTRIBUTION
VALIDITY OF SECURITIES
EXPERTS
PART II
SIGNATURES
EXHIBIT INDEX TO REGISTRATION STATEMENT ON FORM S-3

EX-5

                                                                       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:




Aon Corporation
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




Aon Corporation
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




Aon Corporation
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

EX-23.(B)

                                                                   Exhibit 23(b)

               CONSENT OF ERNST & YOUNG LLP, INDEPENDENT AUDITORS

We consent to the reference to our firm under the caption "Experts" in the
Registration Statement (Form S-3 No. 333-_____) and related Prospectus of Aon
Corporation for the registration of $750,000,000 of debt securities, preferred
stock, common stock, share purchase contacts and share purchase units and to the
incorporation by reference therein of our reports dated February 8, 2001, with
respect to the consolidated financial statements of Aon Corporation
incorporated by reference in its Annual Report (Form 10-K) for the year ended
December 31, 2000 and the related financial statement schedules included
therein, filed with the Securities and Exchange Commission.

                                          /s/ Ernst & Young LLP

Chicago, Illinois
December 3, 2001

EX-23.(C)

                                                                   Exhibit 23(c)

            ACKNOWLEDGMENT OF ERNST & YOUNG LLP, INDEPENDENT AUDITORS

Board of Directors and Stockholders
Aon Corporation

We are aware of the incorporation by reference in the Registration Statement
(Form S-3 No. 333-_____) and related Prospectus of Aon Corporation for the
registration of $750,000,000 of debt securities, preferred stock, common stock,
share purchase contacts and share purchase units of our reports dated May 9,
2001, August 9, 2001, and November 13, 2001 relating to the unaudited condensed
consolidated interim financial statements of Aon Corporation that are included
in its Form 10-Q for the quarters ended March 31, 2001, June 30, 2001 and
September 30, 2001.

                                       /s/ Ernst & Young LLP


December 3, 2001

EX-24

                                                                      Exhibit 24


                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.



                                       /s/ Franklin A. Cole
                                       ---------------------------



                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ Edgar D. Jannotta
                                       -----------------------------







                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ Lester B. Knight
                                       ----------------------------





                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                        /s/ Perry J. Lewis
                                        -----------------------------







                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ Andrew J. McKenna
                                       ---------------------------







                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ Robert S. Morrison
                                       ------------------------------







                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ Richard C. Notebaert
                                       --------------------------------







                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ Michael D. O'Halleran
                                       -------------------------------





                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ Donald S. Perkins
                                       -------------------------------





                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ John W. Rogers, Jr.
                                       -----------------------------





                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ George A. Schaefer
                                       -----------------------------







                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ Fred L. Turner
                                       ---------------------------





                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ Arnold R. Weber
                                       -----------------------------







                                POWER OF ATTORNEY

     KNOW ALL MEN BY THESE PRESENTS, that the person whose signature appears
below constitutes and appoints Patrick G. Ryan, Michael D. O'Halleran, Harvey N.
Medvin and Raymond I. Skilling, and each of them, with full power to act without
the others, his true and lawful attorneys-in-fact and agents, with full and
several power of substitution and resubstitution for the undersigned and in the
undersigned's name, place and stead, in any and all capacities, to sign a
registration statement (the "Universal Registration Statement") under the
Securities Act of 1933, as amended, for the registration of the Debt Securities
and the Capital Stock (which registration may, but shall not be required to be,
made pursuant to Rule 415 under the Securities Act) and any and all amendments
and supplements thereto or to the prospectus contained therein, (including
pre-effective and post-effective amendments) to such Universal Registration
Statement, and to file the same, with all exhibits thereto, and other documents
in connection therewith, with the Securities and Exchange Commission, and any
and all documents relating to the qualification or registration under state Blue
Sky or securities laws of such securities, granting unto such attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite or necessary to be done in and about the
premises, as fully to all intents and purposes as the undersigned might or could
do in person, hereby ratifying and confirming all that said attorneys-in-fact
and agents or any of them, or their or his substitute or substitutes, may
lawfully do or cause to be done by virtue hereof.

     IN WITNESS WHEREOF, the undersigned has signed his/her name effective as of
November 16, 2001.

                                       /s/ Carolyn Y. Woo
                                       -----------------------------

EX-25.(A)

                                                                   Exhibit 25(a)
================================================================================


                                    FORM T-1

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                            STATEMENT OF ELIGIBILITY
                   UNDER THE TRUST INDENTURE ACT OF 1939 OF A
                    CORPORATION DESIGNATED TO ACT AS TRUSTEE

                      CHECK IF AN APPLICATION TO DETERMINE
                      ELIGIBILITY OF A TRUSTEE PURSUANT TO
                             SECTION 305(b)(2) |__|

                           ---------------------------

                              THE BANK OF NEW YORK
               (Exact name of trustee as specified in its charter)

New York                                                  13-5160382
(State of incorporation                                   (I.R.S. employer
if not a U.S. national bank)                              identification no.)

One Wall Street, New York, N.Y.                           10286
(Address of principal executive offices)                  (Zip code)

                           ---------------------------

                                 Aon CORPORATION
               (Exact name of obligor as specified in its charter)

Delaware                                                  39-3051915
(State or other jurisdiction of                           (I.R.S. employer
incorporation or organization)                            identification no.)

200 East Randolph Street
Chicago, Illinois                                         60601
(Address of principal executive offices)                  (Zip code)

                           ---------------------------

                                 Debt Securities
                       (Title of the indenture securities)

================================================================================



1.   GENERAL INFORMATION. FURNISH THE FOLLOWING INFORMATION AS TO THE TRUSTEE:

     (A)  NAME AND ADDRESS OF EACH EXAMINING OR SUPERVISING AUTHORITY TO WHICH
          IT IS SUBJECT.


- --------------------------------------------------------------------------------
           Name                                                 Address
- --------------------------------------------------------------------------------
                                                 
     Superintendent of Banks of the State of         2 Rector Street, New York, N.Y.
     New York                                        10006, and Albany, N.Y. 12203

     Federal Reserve Bank of New York                33 Liberty Plaza, New York, N.Y.
                                                     10045

     Federal Deposit Insurance Corporation           Washington, D.C.  20429

     New York Clearing House Association             New York, New York   10005


     (B)  WHETHER IT IS AUTHORIZED TO EXERCISE CORPORATE TRUST POWERS.

     Yes.

2.   AFFILIATIONS WITH OBLIGOR.

     IF THE OBLIGOR IS AN AFFILIATE OF THE TRUSTEE, DESCRIBE EACH SUCH
     AFFILIATION.

     None.

16.  LIST OF EXHIBITS.

     EXHIBITS IDENTIFIED IN PARENTHESES BELOW, ON FILE WITH THE COMMISSION, ARE
     INCORPORATED HEREIN BY REFERENCE AS AN EXHIBIT HERETO, PURSUANT TO RULE
     7a-29 UNDER THE TRUST INDENTURE ACT OF 1939 (THE "ACT") AND 17 C.F.R.
     229.10(d).

     1.   A copy of the Organization Certificate of The Bank of New York
          (formerly Irving Trust Company) as now in effect, which contains the
          authority to commence business and a grant of powers to exercise
          corporate trust powers. (Exhibit 1 to Amendment No. 1 to Form T-1
          filed with Registration Statement No. 33-6215, Exhibits 1a and 1b to
          Form T-1 filed with Registration Statement No. 33-21672 and Exhibit 1
          to Form T-1 filed with Registration Statement No. 33-29637.)

     4.   A copy of the existing By-laws of the Trustee. (Exhibit 4 to Form T-1
          filed with Registration Statement No. 33-31019.)

     6.   The consent of the Trustee required by Section 321(b) of the Act.
          (Exhibit 6 to Form T-1 filed with Registration Statement No.
          33-44051.)


                                      -2-


     7.   A copy of the latest report of condition of the Trustee published
          pursuant to law or to the requirements of its supervising or examining
          authority.


                                      -3-


                                    SIGNATURE

     Pursuant to the requirements of the Act, the Trustee, The Bank of New York,
a corporation organized and existing under the laws of the State of New York,
has duly caused this statement of eligibility to be signed on its behalf by the
undersigned, thereunto duly authorized, all in The City of New York, and State
of New York, on the 29th day of November, 2001.


                                       THE BANK OF NEW YORK





                                       By: /s/ STACEY POINDEXTER
                                       ----------------------------------
                                       Name: STACEY POINDEXTER
                                       Title: ASSISTANT TREASURER



                                      -4-



                                                                       EXHIBIT 7


- --------------------------------------------------------------------------------

                       Consolidated Report of Condition of

                              THE BANK OF NEW YORK

                    of One Wall Street, New York, N.Y. 10286
                     And Foreign and Domestic Subsidiaries,
a member of the Federal Reserve System, at the close of business March 31, 2001,
published in accordance with a call made by the Federal Reserve Bank of this
District pursuant to the provisions of the Federal Reserve Act.



                                                                             Dollar Amounts
                                                                             In Thousands
                                                                             --------------
                                                                         
ASSETS
Cash and balances due from depository institutions:
   Noninterest-bearing balances and currency and coin............                $2,811,275
   Interest-bearing balances.....................................                 3,133,222
Securities:
   Held-to-maturity securities...................................                   147,185
   Available-for-sale securities.................................                 5,403,923
Federal funds sold and Securities purchased under
   agreements to resell..........................................                 3,378,526
Loans and lease financing receivables:
   Loans and leases held for sale................................                    74,702
   Loans and leases, net of unearned
     income......................................................                37,471,621
   LESS: Allowance for loan and
     lease losses................................................                   599,061
   Loans and leases, net of unearned
     income and allowance........................................                36,872,560
Trading Assets...................................................                11,757,036
Premises and fixed assets (including capitalized
   leases).......................................................                   768,795
Other real estate owned..........................................                     1,078
Investments in unconsolidated subsidiaries and
   associated companies..........................................                   193,126
Customers' liability to this bank on acceptances
   outstanding...................................................                   592,118
Intangible assets................................................
   Goodwill......................................................                 1,300,295
   Other intangible assets.......................................                   122,143
Other assets.....................................................                 3,676,375
                                                                                ===========



Total assets.....................................................               $70,232,359
                                                                                ===========
LIABILITIES
Deposits:
   In domestic offices...........................................               $25,962,242
   Noninterest-bearing...........................................                10,586,346
   Interest-bearing..............................................                15,395,896
   In foreign offices, Edge and Agreement
     subsidiaries, and IBFs......................................                24,862,377
   Noninterest-bearing...........................................                   373,085
   Interest-bearing..............................................                24,489,292
Federal funds purchased and securities sold under
   agreements to repurchase.....................................                  1,446,874
Trading liabilities.............................................                  2,373,361
Other borrowed money:
   (includes mortgage indebtedness and obligations
   under capitalized leases).......                                               1,381,512
Bank's liability on acceptances executed and
   outstanding.................................................                     592,804
Subordinated notes and debentures..............................                   1,646,000
Other liabilities..............................................                   5,373,065
                                                                                -----------
Total liabilities..............................................                 $63,658,235
                                                                                ===========
EQUITY CAPITAL
Common stock...................................................                   1,135,284
Surplus........................................................                   1,008,773
Retained earnings..............................................                   4,426,033
Accumulated other comprehensive income.........................                       4,034
Other equity capital components................................                           0
                                                                                -----------
Total equity capital...........................................                   6,574,124
                                                                                -----------
Total liabilities and equity capital...........................                 $70,232,359
                                                                                ===========



     I, Thomas J. Mastro, Senior Vice President and Comptroller of the
above-named bank do hereby declare that this Report of Condition has been
prepared in conformance with the instructions issued by the Board of Governors
of the Federal Reserve System and is true to the best of my knowledge and
belief.

                                                               Thomas J. Mastro,
                                           Senior Vice President and Comptroller

     We, the undersigned directors, attest to the correctness of this Report of
Condition and declare that it has been examined by us and to the best of our
knowledge and belief has been




prepared in conformance with the instructions issued by the Board of Governors
of the Federal Reserve System and is true and correct.


Thomas A. Renyi                ]
Gerald L. Hassell              ]                Directors
Alan R. Griffith               ]


- --------------------------------------------------------------------------------

EX-25.(B)

                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549

                                   ----------

                                    FORM T-1

                       Statement of Eligibility Under the
                  Trust Indenture Act of 1939 of a Corporation
                          Designated to Act as Trustee


                      U.S. BANK TRUST NATIONAL ASSOCIATION
               (Exact name of Trustee as specified in its charter)

     United States                                                41-0257700
(State of Incorporation)                                      (I.R.S. Employer
                                                             Identification No.)

         U.S. Bank Trust Center
         180 East Fifth Street
          St. Paul, Minnesota                                        55101
(Address of Principal Executive Offices)                          (Zip Code)


                                 AON CORPORATION

             (Exact name of Registrant as specified in its charter)

         Delaware                                                 36-3051915

(State of Incorporation)                                      (I.R.S. Employer
                                                             Identification No.)


     200 East Randolph Street
         Chicago. Illinois                                           60601
(Address of Principal Executive Offices)                          (Zip Code)



                                 DEBT SECURITIES
                       (Title of the Indenture Securities)






                                     GENERAL

1.   GENERAL INFORMATION Furnish the following information as to the Trustee.

     (a) Name and address of each examining or supervising authority to which it
         is subject.
            Comptroller of the Currency
            Washington, D.C.

     (b) Whether it is authorized to exercise corporate trust powers.
            Yes

2.   AFFILIATIONS WITH OBLIGOR AND UNDERWRITERS If the obligor or any
     underwriter for the obligor is an affiliate of the Trustee, describe each
     such affiliation.
            None

     See Note following Item 16.

     Items 3-15 are not applicable because to the best of the Trustee's
     knowledge the obligor is not in default under any Indenture for which the
     Trustee acts as Trustee.

16.  LIST OF EXHIBITS List below all exhibits filed as a part of this statement
     of eligibility and qualification.

     1.  Copy of Articles of Assocation.*

     2.  Copy of Certificate of Authority to Commence Business.*

     3.  Authorization of the Trustee to exercise corporate trust powers
         (included in Exhibits 1 and 2; no separate instrument).*

     4.  Copy of existing By-Laws.*

     5.  Copy of each Indenture referred to in Item 4. Not Applicable.

     6.  The consents of the Trustee required by Section 321(b) of the act.*

     7.  Copy of the latest report of condition of the Trustee published
         pursuant to law or the requirements of its supervising or examining
         authority is incorporated by reference to Registration Number
         333-43278.

     * Incorporated by reference to Registration Number 22-27000.



                                      NOTE

         The answers to this statement insofar as such answers relate to what
persons have been underwriters for any securities of the obligors within three
years prior to the date of filing this statement, or what persons are owners of
10% or more of the voting securities of the obligors, or affiliates, are based
upon information furnished to the Trustee by the obligors. While the Trustee has
no reason to doubt the accuracy of any such information, it cannot accept any
responsibility therefor.


                                    SIGNATURE

         Pursuant to the requirements of the Trust Indenture Act of 1939, the
Trustee, U.S. Bank Trust National Association, an Association organized and
existing under the laws of the United States, has duly caused this statement of
eligibility and qualification to be signed on its behalf by the undersigned,
thereunto duly authorized, and its seal to be hereunto affixed and attested, all
in the City of Saint Paul and State of Minnesota on the 30th day of November,
2001.


                                            U.S. BANK TRUST NATIONAL ASSOCIATION



                                            /s/ Richard Prokosch
                                            ------------------------------------
                                            Richard Prokosch
                                            Vice President



                                    EXHIBIT 6

                                     CONSENT

         In accordance with Section 321(b) of the Trust Indenture Act of 1939,
the undersigned, U.S. BANK TRUST NATIONAL ASSOCIATION hereby consents that
reports of examination of the undersigned by Federal, State, Territorial or
District authorities may be furnished by such authorities to the Securities and
Exchange Commission upon its request therefor.


Dated:  November 30, 2001


                                            U.S. BANK TRUST NATIONAL ASSOCIATION


                                            /s/ Richard Prokosch
                                            ------------------------------------
                                            Richard Prokosch
                                            Vice President