SC TO-I

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SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, D.C. 20549

SCHEDULE TO

(RULE 13e-4)

TENDER OFFER STATEMENT

UNDER SECTION 14(d)(1) OR 13(e)(1) OF THE SECURITIES EXCHANGE ACT OF 1934

Aon Corporation(Name of Subject Company (Issuer) and Filing Person (Offeror))
Options to Purchase Common Stock, $1.00 Par Value Per Share(Title of Class of Securities)
	037389103(CUSIP Number of Class of Securities)
(Underlying Common Stock)
D. Cameron Findlay Executive Vice President and General Counsel Aon Corporation 200 East Randolph Street Chicago, IL 60601 (312) 381-1000(Name, address and telephone number of person authorized to receive notices and communications on behalf of filing person)

With a Copy to:

Terrence R. Brady
Winston & Strawn LLP
35 W. Wacker Drive
Chicago, IL 60601
(312) 558-5600

CALCULATION OF FILING FEE

Transaction valuation*	Amount of filing fee
$15,544,780	$477.23

*

Calculated solely for purposes of determining the filing fee. This amount assumes that options to purchase 942,426 shares of Common Stock of Aon Corporation having an aggregate value of $15,544,780 will be amended pursuant to offer. The aggregate value of such options was calculated based on the lattice-binomial option-pricing model. The amount of the filing fee, calculated in accordance with Rule 0-11(b) of the Securities Exchange Act of 1934, as amended, equals $30.70 per million of the transaction valuation.

o

Check box if any part of the fee is offset as provided by Rule 0-11(a)(2) and identify the filing with which the offsetting fee was previously paid. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.

Amount Previously Paid:	Not applicable.	Filing party:	Not applicable.
Form or Registration No.:	Not applicable.	Date filed:	Not applicable.

o

Check the box if the filing relates solely to preliminary communications made before the commencement of a tender offer.

Check the appropriate boxes below to designate any transactions to which the statement relates:

o

third party tender offer subject to Rule 14d-1.

ý

issuer tender offer subject to Rule 13e-4.

o

going-private transaction subject to Rule 13e-3.

o

amendment to Schedule 13D under Rule 13d-2.

Check the following box if the filing is a final amendment reporting the results of the tender offer: o

INTRODUCTORY STATEMENT

        This Tender Offer Statement on Schedule TO relates to an offer (the "Offer") by Aon Corporation, a Delaware corporation ("Aon"), to permit its eligible employees to amend certain options (the "Eligible Options") that were granted by Aon under the Aon Stock Option Plan (as amended, the "Aon Stock Option Plan") and the Aon Stock Incentive Plan (as amended, the "Aon Stock Incentive Plan") The Aon Stock Option Plan and the Aon Stock Incentive Plan are referred to collectively herein as the "Plans." The Eligible Options include only those stock options granted by Aon: (i) that were granted under the Plans; (ii) that have an exercise price per share that was less than the fair market value per share of Aon common stock underlying the options on the options' measurement dates for accounting purposes (the "deemed grant date"); (iii) that were unvested as of December 31, 2004 (if a portion of an option was unvested as of December 31, 2004, only the unvested portion is eligible for this Offer); (iv) that are outstanding as of the last date on which the Offer remains open for acceptance, and (v) that are held by individuals who are subject to taxation in the United States.

        An "Eligible Employee" refers to an individual who holds Eligible Options to purchase shares of Aon common stock that are outstanding on the last date on which this Offer remains open for acceptance and who is subject to taxation in the United States. Current and former directors and executive officers of Aon are not Eligible Employees.

        Eligible Employees may elect to: (i) amend all (but not a portion) of their Eligible Options to increase the exercise price per share to be equal to the fair market value of a share of Aon common stock on the deemed grant date (the "Amended Exercise Price"); and (ii) for each amended Eligible Option, receive a cash payment equal to the difference between the Amended Exercise Price and the original exercise price, multiplied by the number of shares of Aon common stock subject to such unexercised option. The cash payments will by paid on the first regular payroll date in January 2008, and will be subject to applicable tax withholding. The cash payments will be made without regard to whether the Eligible Option is vested or whether an eligible participant is an employee of Aon or its subsidiaries at the time of payment.

        The amendment of the Eligible Options will be made pursuant to the terms and subject to the conditions set forth in: (i) the Offer to Amend the Exercise Price of Certain Outstanding Options, dated August 15, 2007 (the "Offer to Amend"); (ii) the related memorandum from Jeremy Farmer, Senior Vice President and Head of Human Resources, dated August 15, 2007; (iii) the Election Form; and (iv) the Withdrawal Form. These documents, as they may be amended or supplemented from time to time, together constitute the "Offer Documents" and are attached to this Schedule TO as Exhibits (a)(1)(a) through (a)(1)(d), respectively.

        This Offer is being made upon the terms and subject to the conditions set forth in the Offer to Amend, which, as may be amended or supplemented from time to time, constitutes the Offer, and which is filed as Exhibit (a)(1)(A) hereto. This Tender Offer Statement on Schedule TO is intended to satisfy the reporting requirements of Section 13(e) of the Securities Exchange Act of 1934, as amended.

        The information in the Offer Documents, including all schedules and annexes to the Offer Documents, is incorporated by reference in answer to the items required in this Schedule TO.

ITEM 1. SUMMARY TERM SHEET.

        The information set forth under the caption "Summary Term Sheet and Questions and Answers" in the Offer to Amend is incorporated herein by reference.

2

ITEM 2. SUBJECT COMPANY INFORMATION.

(a)

Name and Address.    Aon is the issuer of the securities subject to the Offer to Amend. The address and telephone number of Aon's principal executive offices is 200 East Randolph Street, Chicago, IL 60601, (312) 381-1000. The information set forth in the Offer to Amend under the caption "The Offer" in Section 10 ("Information concerning Aon; Summary financial information") is incorporated herein by reference.

(b)

Securities.    The subject class of securities to which this Tender Offer Statement on Schedule TO relates consists of the Eligible Options. As of August 9, 2007, options to purchase a total of 942,426 shares of common stock are Eligible Options. The actual number of shares of common stock subject to the amended options to be issued in the Offer will depend on the number of shares of such common stock subject to the unexercised options tendered by Eligible Employees. The information set forth in the Offer to Amend under the captions "Summary Term Sheet and Questions and Answers," "Risks of Participating in the Offer," and under the caption "The Offer" in Section 2 ("Number of options and amount of Option Consideration; expiration date"), Section 6 ("Acceptance of options for amendment and issuance of cash payments and amended options") and Section 9 ("Source and amount of consideration; terms of amended options") is incorporated herein by reference.

(c)

Trading Market and Price.    The information set forth in the Offer to Amend under the caption "The Offer" in Section 8 ("Price range of shares underlying the options") is incorporated herein by reference.

ITEM 3. IDENTITY AND BACKGROUND OF FILING PERSON

(a)

Name and Address.    The information set forth under Item 2(a) above is incorporated herein by reference. Aon is both the "filing person" and the "subject company". The information set forth in the Offer to Amend under the caption "The Offer" in Section 11 ("Interests of Directors and executive officers; transactions and arrangements concerning the options") is incorporated herein by reference.

ITEM 4. TERMS OF THE TRANSACTION

(a)

Material Terms.    The information set forth in the Offer to Amend under the caption "Summary Term Sheet and Questions and Answers," and under the caption "The Offer" in Section 1 ("Eligibility"), Section 2 ("Number of options and amount of Option Consideration; expiration date"), Section 4 ("Procedures for electing to participate in this offer"), Section 5 ("Withdrawal rights and change of election"), Section 6 ("Acceptance of options for amendment and issuance of cash payments and amended options"), Section 7 ("Conditions of the offer"), Section 8 ("Price range of shares underlying the options"), Section 9 ("Source and amount of consideration; terms of amended options"), Section 12 ("Status of options amended by us in the offer; accounting consequences of the offer"), Section 13 ("Legal matters; regulatory approvals"), Section 14 ("Material United States federal income tax consequences"), and Section 15 ("Extension of offer; termination; amendment") is incorporated herein by reference.

(b)

Purchases.    No current or former member of Aon's Board of Directors or current or former executive officer of Aon may participate in the Offer. The information set forth in the Offer to Amend under the caption "The Offer" in Section 11 ("Interests of Directors and executive officers; transactions and arrangements concerning the options") is incorporated herein by reference.

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ITEM 5. PAST CONTACTS, TRANSACTIONS, NEGOTIATIONS AND AGREEMENTS

(e)

Agreements Involving the Subject Company's Securities.    The information set forth in the Offer to Amend under the caption "The Offer" in Section 11 ("Interests of Directors and executive officers; transactions and arrangements concerning the options") and Section 9 ("Source and amount of consideration; terms of amended options") is incorporated herein by reference. The Aon Stock Incentive Plan and Aon Stock Option Plan, pursuant to which the Eligible Options have been granted are attached as exhibits hereto and are incorporated herein by reference.

ITEM 6. PURPOSES OF THE TRANSACTION AND PLANS OR PROPOSALS

(a)

Purposes.    The information set forth in the Offer to Amend under the caption "Summary Term Sheet and Questions and Answers" and under the caption "The Offer" in Section 3 ("Purpose of the offer") is incorporated herein by reference.

(b)

Use of Securities Acquired.    The information set forth in the Offer to Amend under the caption "The Offer" in Section 6 ("Acceptance of options for amendment and issuance of cash payments and amended options") and Section 12 ("Status of options amended by us in the offer; accounting consequences of the offer") is incorporated herein by reference.

(c)

Plans.    The information set forth in the Offer to Amend under the caption "The Offer" in Section 3 ("Purpose of the offer") and Section 10 ("Information concerning Aon; Summary financial information") is incorporated herein by reference.

ITEM 7. SOURCE AND AMOUNT OF FUNDS OR OTHER CONSIDERATION

(a)

Source of Funds.    The information set forth in the Offer to Amend under the caption "The Offer" in Section 9 ("Source and amount of consideration; terms of amended options") and Section 16 ("Fees and expenses") is incorporated herein by reference.

(b)

Conditions.    Not applicable.

(d)

Borrowed Funds.    Not applicable.

ITEM 8. INTEREST IN SECURITIES OF THE SUBJECT COMPANY

(a)

Securities Ownership.    The information set forth in the Offer to Amend under the caption "The Offer" in Section 11 ("Interests of Directors and executive officers; transactions and arrangements concerning the options") is incorporated herein by reference.

(b)

Securities Transactions.    The information set forth in the Offer to Amend under the caption "The Offer" in Section 11 ("Interests of Directors and executive officers; transactions and arrangements concerning the options") is incorporated herein by reference.

ITEM 9. PERSON/ASSETS, RETAINED, EMPLOYED, COMPENSATED OR USED

(a)

Solicitations and Recommendations.    Not applicable.

ITEM 10. FINANCIAL STATEMENTS

(a)

Financial Information.    The information set forth in Item 8, Consolidated Financial Statements and Supplementary Data, Exhibit 12(a) and Exhibit 12(b) of the Aon's Annual Report on Form 10-K for the year ended December 31, 2006, filed with the Securities and Exchange Commission (the "SEC") on March 1, 2007, including all material incorporated by reference therein, is incorporated herein by reference. Item 1, Financial Statements, Exhibit 12(a) and Exhibit 12(b) of Aon's Quarterly Report on Form 10-Q for the quarter ended March 31, 2007, filed with the SEC on

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May 10, 2007, including all material incorporated by reference therein, is incorporated herein by reference. Item 1, Financial Statements, Exhibit 12(a) and Exhibit 12(b) of Aon's Quarterly Report on Form 10-Q for the quarter ended June 30, 2007, filed with the SEC on August 9, 2007, including all material incorporated by reference therein, is incorporated herein by reference. Aon's Annual Reports on 10-K and Quarterly Report on Form 10-Q can be accessed electronically on the SEC's website at http://www.sec.gov.

(b)

Pro Forma Information.    Not applicable.

(c)

Summary Information.    The information set forth in the Offer to Amend under the caption "The Offer" in the section entitled "Information concerning Aon; Summary financial information" (Section 10) is incorporated herein by reference.

ITEM 11. ADDITIONAL INFORMATION.

(a)

Agreements, Regulatory Requirements and Legal Proceedings.

(1)

The information set forth in the Offer to Amend under the caption "The Offer" in Section 11 ("Interests of Directors and executive officers; transactions and arrangements concerning the options") is incorporated herein by reference.

(2)

The information set forth in the Offer to Amend under the caption Section 13 ("Legal matters; regulatory approvals") is incorporated herein by reference.

(3)

Not Applicable.

(4)

Not Applicable.

(5)

Not Applicable.

(b)

Other Material Information.    The information set forth in the Offer to Amend under the caption "Additional Information" in Section 17 is incorporated herein by reference.

ITEM 12. EXHIBITS.

(a)	(1)(a)	Offer to Amend the Exercise Price of Certain Options, dated August 15, 2007.
	(1)(b)	Email to Eligible Option Holders from Danita E. Dallman, dated August 10, 2007—incorporated by reference to Exhibit (a)(1)(b) to Aon's Tender Offer Statement on Schedule 13D filed on August 13, 2007.*
	(1)(c)	Memorandum to Eligible Option Holders from Jeremy Farmer, Senior Vice President and Head of Human Resources, dated August 15, 2007.
	(1)(d)	Form of Election Form.
	(1)(e)	Form of Withdrawal Form.
	(1)(f)	Form of Promise to Make Cash Payment.
	(1)(g)	Forms of Confirmation E-mails.
	(1)(h)	Forms of Reminder E-mail Communications to Eligible Employees.
	(1)(i)	Form of Amendment to Stock Option Agreements.
	(1)(j)	Employee Presentation Materials.
(b)	None.

5

(d)	(1)	Aon Stock Option Plan (as amended and restated through 1997)—incorporated by reference to Exhibit 10(a) to Aon's Quarterly Report on Form 10-Q for the quarter ended March 31, 1997.*
	(2)	First Amendment to Aon Stock Option Plan (as amended and restated through 1997)—incorporated by reference to Exhibit 10(a) to Aon's Quarterly Report on Form 10-Q for the quarter ended March 31, 1999.*
	(3)	Second Amendment to Aon Stock Option Plan (as amended and restated through 1997).
	(4)	Third Amendment to Aon Stock Option Plan (as amended and restated through 1997)—incorporated by reference to Exhibit 10(at) to Aon's Annual Report on Form 10-K for the year ended December 31, 2006.*
	(5)	Aon Stock Incentive Plan (as amended and restated through May 2006)—incorporated by reference to Exhibit 10.2 to Aon's Current Report on Form 8-K filed on May 24, 2006.*
	(6)	First Amendment to Aon Stock Incentive Plan (as amended and restated through May 2006)—incorporated by reference to Exhibit 10(au) to Aon's Annual Report on Form 10-K for the year ended December 31, 2006.*
	(7)	Form of Stock Option Award Agreement.
(g)	None.
(h)	None.

*

Document has heretofore been filed with the Securities and Exchange Commission and is incorporated by reference and made a part hereof.

ITEM 13. INFORMATION REQUIRED BY SCHEDULE 13E-3

(a)

Not applicable.

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SIGNATURE

        After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this Schedule TO is true, complete and correct.

	AON CORPORATION
	By:	/s/  D. CAMERON FINDLAY D. CAMERON FINDLAY EXECUTIVE VICE PRESIDENT AND GENERAL COUNSEL
Date: August 15, 2007

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EXHIBIT INDEX

Exhibit Number	Description
(a)(1)(a)	Offer to Amend the Exercise Price of Certain Options, dated August 15, 2007.
(a)(1)(b)	Email to Eligible Option Holders from Danita E. Dallman, dated August 10, 2007—incorporated by reference to Exhibit (a)(1)(b) to Aon's Tender Offer Statement on Schedule 13D filed on August 13, 2007.*
(a)(1)(c)	Memorandum to Eligible Option Holders from Jeremy Farmer, Senior Vice President and Head of Human Resources, dated August 15, 2007.
(a)(1)(d)	Form of Election Form.
(a)(1)(e)	Form of Withdrawal Form.
(a)(1)(f)	Form of Promise to Make Cash Payment.
(a)(1)(g)	Forms of Confirmation E-mails.
(a)(1)(h)	Forms of Reminder E-mail Communications to Eligible Employees.
(a)(1)(i)	Form of Amendment to Stock Option Agreements.
(a)(1)(j)	Employee Presentation Materials.
(d)(1)	Aon Stock Option Plan (as amended and restated through 1997)—incorporated by reference to Exhibit 10(a) to Aon's Quarterly Report on Form 10-Q for the quarter ended March 31, 1997.*
(d)(2)	First Amendment to Aon Stock Option Plan (as amended and restated through 1997)—incorporated by reference to Exhibit 10(a) to Aon's Quarterly Report on Form 10-Q for the quarter ended March 31, 1999.*
(d)(3)	Second Amendment to Aon Stock Option Plan (as amended and restated through 1997).
(d)(4)	Third Amendment to Aon Stock Option Plan (as amended and restated through 1997)—incorporated by reference to Exhibit 10(at) to Aon's Annual Report on Form 10-K for the year ended December 31, 2006.*
(d)(5)	Aon Stock Incentive Plan (as amended and restated through May 2006)—incorporated by reference to Exhibit 10.2 to Aon's Current Report on Form 8-K filed on May 24, 2006.*
(d)(6)	First Amendment to Aon Stock Incentive Plan (as amended and restated through May 2006)—incorporated by reference to Exhibit 10(au) to Aon's Annual Report on Form 10-K for the year ended December 31, 2006.*
(d)(7)	Form of Stock Option Award Agreement.

*

Document has heretofore been filed with the Securities and Exchange Commission and is incorporated by reference and made a part hereof.

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INTRODUCTORY STATEMENT

ITEM 1. SUMMARY TERM SHEET.
ITEM 2. SUBJECT COMPANY INFORMATION.
ITEM 3. IDENTITY AND BACKGROUND OF FILING PERSON
ITEM 4. TERMS OF THE TRANSACTION
ITEM 5. PAST CONTACTS, TRANSACTIONS, NEGOTIATIONS AND AGREEMENTS
ITEM 6. PURPOSES OF THE TRANSACTION AND PLANS OR PROPOSALS
ITEM 7. SOURCE AND AMOUNT OF FUNDS OR OTHER CONSIDERATION
ITEM 8. INTEREST IN SECURITIES OF THE SUBJECT COMPANY
ITEM 9. PERSON/ASSETS, RETAINED, EMPLOYED, COMPENSATED OR USED
ITEM 10. FINANCIAL STATEMENTS
ITEM 11. ADDITIONAL INFORMATION.
ITEM 12. EXHIBITS.
ITEM 13. INFORMATION REQUIRED BY SCHEDULE 13E-3

SIGNATURE
EXHIBIT INDEX

EX-99.(A)(1)(A)

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Exhibit (a)(1)(a)

AON CORPORATION

OFFER TO AMEND THE EXERCISE PRICE OF CERTAIN OPTIONS

Any questions or requests for assistance or additional copies of any documents referred to in this
offer may be directed to the Aon Tender Offer Call Center, at 1-888-295-1814

This document constitutes part of the prospectus relating to each of the Aon Stock Option Plan, as amended, and the Aon Stock Incentive Plan, as amended to date,
covering securities that have been registered under the Securities Act of 1933, as amended.

        We are not making this offer to, nor will we accept any election to amend options by or on behalf of, option holders in any jurisdiction in which this offer or the acceptance of any election to amend options would not be in compliance with the laws of such jurisdiction. However, we may, at our discretion, take any actions necessary or desirable for us to make this offer to option holders in any jurisdiction.

August 15, 2007

AON CORPORATION

Offer to Amend the Exercise Price of Certain Options

This offer and your participation and withdrawal rights will expire at
11:59 p.m., Chicago Time, on Wednesday, September 17, 2007 unless we extend them.

        By this Offer to Amend the Exercise Price of Certain Options (the "Offer to Amend"), we are giving all eligible employees holding outstanding eligible options to purchase shares of our common stock ("options") the right to amend such options and to receive cash payments, as described below (we refer to this collectively as the "offer"). Each eligible employee holding eligible options is being provided with a personalized election form (referred to as the "election form") setting forth his or her eligible options, the new exercise price that would apply to each eligible option (if amended) and a description of any potential cash payments. We refer to the amended options and/or cash payments a participant may receive in replacement of his or her eligible options as the "Option Consideration."

        Based upon a review of option grant practices by the Audit Committee of the Board of the Directors, we have determined that certain stock options were granted at a discount from fair market value and therefore may be subject to adverse tax consequences under Section 409A of the Internal Revenue Code of 1986, as amended (such options are referred to as "affected options"). In addition, some states may impose additional penalty taxes. These consequences include ordinary income tax at vesting, an additional 20% penalty tax and interest charges. If you elect to participate in this Offer to Amend, your amended options will no longer be subject to the adverse tax consequences of Section 409A, although your options may still be subject to adverse state tax consequences.

        You are an "eligible employee" only if you are an employee of Aon Corporation or one of our subsidiaries (collectively referred to as "Aon," the "Company," "we," "our" or "us") holding eligible options to purchase shares of Aon common stock that are outstanding on the last date on which this offer remains open for acceptance and who is subject to taxation in the United States. Current and former directors and executive officers of Aon are not eligible employees.

        An option to purchase shares of Aon common stock is eligible for this offer (an "eligible option") only if each of the following conditions is met:

•

the option was granted under the Aon Stock Option Plan (as amended, the "Aon Stock Option Plan"), or the Aon Stock Incentive Plan, (as amended, the "Aon Stock Incentive Plan" and together with the Aon Stock Option Plan, the "Plans");



•

the option has an exercise price per share that was less than the fair market value per share of Aon common stock underlying the option on the option's measurement date for accounting purposes (the "deemed grant date");



•

the option was unvested as of December 31, 2004 (if a portion of an option was unvested as of December 31, 2004, only the unvested portion is eligible for this offer);



•

the option is outstanding as of the last date on which this offer remains open for acceptance; and



•

the option is held by an eligible employee.

2

        You should note that any options (or portion of the options) that vested on or before December 31, 2004 are not subject to additional tax under Section 409A, and are therefore not included in this offer and will retain the original terms under which they were granted.

        This is an all or nothing offer, which means that if you participate, you must participate with respect to all of your eligible options. If you participate in this offer, all of your eligible options will be replaced with the following consideration (the "Option Consideration"), which is detailed more fully in Section 2 of this Offer to Amend:

1.

First, your eligible options will be amended to increase the exercise price per share to be equal to the fair market value of a share of Aon common stock on the deemed grant date (the "amended exercise price"). If only a portion of your option was unvested as of December 31, 2004, then only that portion of the option is an eligible option and will be amended to increase the exercise price. Your election form lists the original exercise price of your eligible options (referred to as the "original exercise price"), as well as the new exercise price of such options, should you accept this offer in connection with those options; and



2.

For each such eligible option amended in this offer, you will receive a cash payment equal to the difference between the amended exercise price and the original exercise price, multiplied by the number of shares of Aon common stock subject to such unexercised option. The cash payments will by paid on the first regular payroll date in January 2008, and will be subject to applicable tax withholding. The cash payments will be made without regard to whether an Eligible Employee is an employee of Aon or its subsidiaries on the cash payment date.

If you do not accept this offer, you will not receive amended options or any cash payment, and your affected options will remain subject to adverse tax consequences under Section 409A, including ordinary income tax at vesting, an additional 20% penalty tax and interest charges, and we will not reimburse you for any such taxes or any related penalties, charges or fees.

Receipt of Amended Options and Cash Payments

        If you elect to receive an amended option for an eligible option, such option will be amended on the next business day following the date that this offer expires and you will receive paperwork regarding your amended option promptly after the expiration of the offer. That amendment date is currently expected to be September 18, 2007. Promptly following the expiration of the offer, we will send you an "Amendment to Stock Option Agreements," which you will be asked to promptly sign and return to us. Each amended option will be subject to this agreement between you and Aon. Any amended option you receive will continue to be subject to the same vesting schedule as your original eligible option.

        Promptly following the expiration of the offer, we will send you a "Promise to Make Cash Payment" evidencing your right to receive a cash payment for your eligible options. Cash payments will be paid to you on the first regular payroll date in January 2008 and will be subject to applicable tax withholding. Cash payments will not be subject to any further vesting conditions, so you will receive any cash payments to which you are entitled, regardless of whether the eligible option is vested and regardless of whether you are an employee of Aon at the time of payment.

Other Matters

        The offer is not conditioned upon acceptance by a minimum number of participants or a minimum number of outstanding eligible options, but the offer is subject to customary conditions, which we describe in Section 7 of this Offer to Amend. You are not required to accept this offer.

        Our common stock is traded on the New York Stock Exchange ("NYSE") under the symbol "AOC." On August 9, 2007, the closing price of our common stock was $41.46 per share. You should evaluate current market quotes for our common stock, among other factors, before deciding to participate in this offer.

See "Risks of Participating in the Offer" beginning on page 19 for a discussion of risks that you should consider before participating in this offer.

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IMPORTANT—STEPS YOU MUST TAKE TO PARTICIPATE

        Today we are sending you an e-mail message containing your election form, which you must complete, sign and submit electronically through Lotus Notes. If you wish to participate in this offer, you must submit a properly completed and electronically signed election form through Lotus Notes prior to 11:59 p.m., Chicago Time, on September 17, 2007. Only election forms that are properly completed and submitted in this manner through Lotus Notes by the expiration date will be accepted. Election forms that are submitted after the expiration date will not be accepted. Aon intends to confirm the receipt of your election form and/or any withdrawal form by an e-mail message to you within two business days. If you have not received an e-mail confirmation, you should confirm that we have received your election form and/or any withdrawal form. Election forms submitted by any means other than that described above are not permitted without the prior consent of Jeremy Farmer, Senior Vice President and Head of Human Resources.

        Neither the Securities and Exchange Commission (the "SEC") nor any state securities commission has approved or disapproved of these securities or passed upon the accuracy or adequacy of this offer. Any representation to the contrary is a criminal offense.

        You should direct questions about the terms of this offer to:

Aon Tender Offer Call Center
1-888-295-1814

        The hours of operation of the Aon Tender Offer Call Center are: 8:00 a.m. to 5:00 p.m. Chicago Time.

Offer to Amend the Exercise Price of Certain Options dated August 15, 2007.

        You should rely only on the information contained in this Offer to Amend or documents to which we have referred you. We have not authorized anyone to provide you with different information. We recommend that you consult with your financial, legal and/or tax advisors regarding any tax consequences, including any state tax consequences. Although our Board of Directors has approved this offer, neither we nor our Board of Directors make any recommendation as to whether you should accept this offer. The decision to participate in the offer must be your own, after taking into account your personal circumstances and preferences. We are not making any offer to amend options in any jurisdiction in which the offer is not permitted. However, we may, at our discretion, take any actions necessary for us to make the offer to option holders in any of these jurisdictions.

        You should not assume that the information provided in this Offer to Amend is accurate as of any date other than the date as of which it is shown, or if no date is otherwise indicated, the date of this offer. Notwithstanding the prior sentence, we will update this Offer to Amend during the offer period if there is a material change in the information presented. This Offer to Amend summarizes various documents and other information. These summaries are qualified in their entirety by reference to the documents and information to which they relate.

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TABLE OF CONTENTS

Summary Term Sheet and Questions and Answers			6
Risks of Participating in the Offer			19
The Offer			21
	1.	Eligibility	21
	2.	Number of options and amount of Option Consideration; expiration date	21
	3.	Purpose of the offer	23
	4.	Procedures for electing to participate in this offer	24
	5.	Withdrawal rights and change of election	25
	6.	Acceptance of options for amendment and issuance of cash payments and amended options	26
	7.	Conditions of the offer	28
	8.	Price range of shares underlying the options	30
	9.	Source and amount of consideration; terms of amended options	30
	10.	Information concerning Aon; summary financial information	34
	11.	Interests of directors and executive officers; transactions and arrangements concerning the options	36
	12.	Status of options amended by us in the offer; accounting consequences of the offer	36
	13.	Legal matters; regulatory approvals	36
	14.	Material United States federal income tax consequences	36
	15.	Extension of offer; termination; amendment	38
	16.	Fees and expenses	39
	17.	Additional information	39
	18.	Miscellaneous	40
Schedule A—Information concerning the directors and executive officers of Aon Corporation			A-1

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SUMMARY TERM SHEET AND QUESTIONS AND ANSWERS

        The following are answers to some of the questions that you may have about this offer. You should carefully read this entire offer, the accompanying memorandum from Jeremy Farmer, Senior Vice President and Head of Human Resources, dated August 15, 2007, and the election and withdrawal forms together with their associated instructions. This offer is made subject to the terms and conditions of these documents as they may be amended. The information in this summary is not complete. Additional important information is contained in the remainder of this Offer to Amend and the other offer documents. We have included in this summary references to other sections in this offer to help you find a more complete description of these topics.

Q1.	What is the offer?
Q2.	What do some of the terms used in this offer mean?
Q3.	Why is Aon making this offer?
Q4.	What is Section 409A?
Q5.	What happens if options are deemed to be deferred compensation under Section 409A?
Q6.	Who is eligible to participate in this offer?
Q7.	Which options are eligible for the Option Consideration in this offer?
Q8.	How do I participate in this offer?
Q9.	If I decide to participate in the offer, what will happen to my current eligible options?
Q10.	What will I receive in return for my eligible options?
Q11.	When will I receive my cash payments and amended options?
Q12.	Why do I have to wait until 2008 to receive a cash payment?
Q13.	Am I required to participate in this offer?
Q14.	What will be the exercise price of my amended options?
Q15.	Once my options are accepted, is there anything I must do to receive the cash payments or amended options?
Q16.	When will my amended options vest?
Q17.	Will the terms and conditions of my amended options be the same as my original options?
Q18.	What happens to my options if I do not turn in my election form by the expiration date, choose not to participate or my options are not accepted?
Q19.	If I hold multiple eligible options, can I choose the options for which I want to accept this offer?
Q20.	If I choose to participate in this offer, are there circumstances under which my eligible options would be amended and I would not receive a cash payment?
Q21.	How will Aon confirm to me that my election form or withdrawal form has been received?
Q22.	Can I accept this offer with respect to shares of Aon common stock that I previously acquired upon exercise of Aon options?
Q23.	Will my decision to participate in the offer have an impact on my ability to receive options in the future?

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Q24.	Is this a modification of options?
Q25.	Will the amendment of my option change the tax treatment of my option?
Q26.	How does Aon determine whether I have properly accepted this offer?
Q27.	When will my amended options expire?
Q28.	Will I receive an amended option agreement?
Q29.	Are there any conditions to this offer?
Q30.	If you extend the offer, how will you notify me?
Q31.	How will you notify me if the offer is changed?
Q32.	Can I change my mind and withdraw from the offer?
Q33.	How do I withdraw my election?
Q34.	What if I withdraw my election and then decide again that I want to participate in this offer?
Q35.	How should I decide whether or not to accept this offer with respect to my eligible options?
Q36.	What happens to my options if my employment with Aon terminates before the offer expires?
Q37.	What happens to my options if my employment with Aon terminates after the expiration of the offer?
Q38.	Does Section 409A impact shares purchased pursuant to the Employee Stock Purchase Plan?
Q39.	Whom can I talk to if I have questions about the offer, or if I need additional copies of the offer documents?

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Q1.    What is the offer?

A1.    This offer is a voluntary opportunity for eligible employees to elect to have certain outstanding options amended and, for such amended options, receive a deferred cash payment. This opportunity is described in the following questions and answers, and in the remainder of this Offer to Amend. We believe this will eliminate certain unfavorable personal tax consequence to eligible employees.

Q2.    What do some of the terms used in this offer mean?

A2.    The following are some terms that are frequently used in this Offer to Amend.

•

"affected options" refers to those stock options granted by Aon that have been identified as having an exercise price below the fair market value of the underlying Aon common stock on the deemed grant date and therefore may be subject to adverse tax consequences under Section 409A of the Internal Revenue Code of 1986, as amended (the "Internal Revenue Code").



•

"amended options" refers to eligible options that are amended pursuant to this offer.



•

"amendment date" refers to the date when eligible options with respect to which you accept this offer will be amended to reflect a new exercise price, which will be equal to the fair market value of a share of our common stock on the eligible option's deemed grant date. We expect that the amendment date will be September 18, 2007, which is the next business day following the expiration date of this offer. If the expiration date is extended, then the amendment date will be similarly extended. Please note that, in order to process these option amendments with E*TRADE, your amended options may not be exercisable for up to three business days following the amendment date.



•

"business day" means any day other than a Saturday, Sunday or a United States federal holiday, and shall consist of the time period from 12:01 a.m. to 12:00 midnight Eastern Time.



•

"deemed grant date" refers to the measurement date of an option for accounting purposes.



•

"election form" refers to the document that will be provided to each eligible employee who holds eligible options. The election form will list eligible options, the new exercise price that would apply to each eligible option (if amended) and a description of any potential cash payments. Eligible employees will receive a personalized election form by e-mail promptly after commencement of this offer.



•

"eligible employee" refers to all current employees of Aon Corporation or our subsidiaries as of the last date on which this offer remains open for acceptance who hold eligible options. Only employees who are subject to taxation in the United States are eligible employees. Current and former directors and executive officers of Aon are not eligible employees.



•

"eligible options" refers to all options with respect to which you may accept this offer in exchange for the Option Consideration, as described in Question and Answer 7 and Sections 1 and 2 of the Offer to Amend.



•

"employee" means a full-time or part-time employee of Aon Corporation or our subsidiaries.



•

"fair market value" with respect to shares of our common stock on a given date, means the closing price of our common stock on the NYSE on such date.



•

"expiration date" refers to the date that this offer expires. The expiration date will be September 17, 2007 at 11:59 p.m., Chicago Time, unless the offer is extended. We may extend the expiration date at our sole discretion. If we extend the offer, the term "expiration date" will refer to the time and date at which the extended offer expires.

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•

"offer period" or "offering period" refers to the period from the commencement of this offer to the expiration date. This period will commence on August 15, 2007, and end at 11:59 p.m., Chicago Time, on September 17, 2007, unless the offer is extended.



•

"Option Consideration" means the total consideration you may receive for your eligible options. The Option Consideration is more fully described in Question and Answer 7 and in Section 6 of this Offer to Amend.



•

"trading day" refers to each day on which the NYSE is open for trading.

Q3.    Why is Aon making this offer?

A3.    We have determined that certain stock options were granted by Aon at a discount from fair market value and therefore may be subject to adverse tax consequences under Section 409A of the Internal Revenue Code. These consequences include ordinary income tax at vesting, an additional 20% penalty tax and interest charges.

        The Offer is being made to eligible employees to address these potential adverse personal tax consequences. By amending such options, the eligible employee should be able to minimize or avoid the application of such adverse federal tax treatment, which is described in Question and Answer 5 and Section 14 of this Offer to Amend. We make no representations as to the effect of this offer under Section 409A or the effect of the offer under state tax laws that are similar to Section 409A.

Q4.    What is Section 409A?

A4.    Effective January 1, 2005, Section 409A of the Internal Revenue Code was added by the American Jobs Creation Act of 2004 (the "AJCA") to make changes in the laws relating to deferred compensation. Under the AJCA, deferred compensation includes stock options with an exercise price that is less than the fair market value of the underlying common stock on the grant date to the extent such options were unvested as of December 31, 2004.

Q5. What happens if options are deemed to be deferred compensation under Section 409A?

A5.    Under the final regulations adopted by the Internal Revenue Service ("IRS") regarding how options which are deemed to be deferred compensation are treated under Section 409A, we believe that the following adverse U.S. federal tax consequences could apply:

•

The optionholder would generally recognize taxable income in the tax year(s) when the option vests and when the option is exercised. The amount of income recognized in connection with vesting will likely be equal to the fair market value of the newly vested shares, less the exercise price payable for those shares and less any income previously recognized. The amount of income recognized in connection with the exercise of the option will likely be equal to the fair market value of the purchased shares, less the sum of the exercise price and any income previously recognized.



•

The optionholder would incur an additional twenty percent (20%) tax on the income recognized in connection with the above paragraph.



•

The optionholder may also be liable for additional tax in the nature of interest.

        We believe that taxation could occur in such manner even though the option remains unexercised. It is also possible that the optionholder may be subject to additional annual income taxes and penalty taxes described above, on any increase in the value of the option shares that has not previously been recognized as income, until the option is exercised or expires. While it is not clear how any future annual increases in value would be measured, such taxation would likely be based on the value of the shares on December 31 of the applicable year.

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        In addition, certain states have also adopted laws similar to Section 409A, and an optionholder may also incur additional taxes and penalties under such state law provisions with respect to discounted options based on the state in which he or she is subject to taxation. For instance, California has adopted a provision similar to Section 409A that could result in an aggregate state and federal tax rate of approximately 80% with regard to a discounted stock option for individuals subject to taxation in California.

        You should consult with your personal financial, tax and legal advisors with regard to the impact of Section 409A (and applicable state tax laws) on your eligible options. Please note, however, that Aon is currently making its own determination as to whether the eligible options are subject to Section 409A. If, based on the available facts, Aon makes a determination that the eligible options are more likely than not subject to Section 409A, Aon will report to the IRS (and any applicable state taxing authorities), and make applicable tax withholdings in respect of, any income that should be recognized by you under Section 409A in connection with those eligible options that are not amended in the offer, as provided by applicable law. You are solely responsible for any income and excise taxes, penalties, and interest payable under Section 409A and other federal, state and foreign tax laws.

Q6.    Who is eligible to participate in this offer?

A6.    You may participate in this offer if you are an employee of Aon or one of our subsidiaries on the last date on which this offer remains open for acceptance. You must hold eligible options in order to participate in this offer. Only employees subject to taxation in the United States hold eligible options. (See Section 1)

        Current and former directors and executive officers of Aon are not eligible to participate in this offer.

Q7.    Which options are eligible for the Option Consideration in this offer?

A7.    An option to purchase common stock is an eligible option under this offer only if each of the following conditions is met:

•

the option was granted under the Aon Stock Option Plan or the Aon Stock Incentive Plan;



•

the option has an exercise price per share that was less than the fair market value per share of Aon common stock underlying the option on the option's deemed grant date;



•

the option was unvested as of December 31, 2004 (if a portion of an option was unvested as of December 31, 2004, only the unvested portion is eligible for this offer);



•

the option is outstanding as of the last date on which this offer remains open for acceptance; and



•

the option is held by an eligible employee.

        The tax consequences of Section 409A will only affect that portion of an option that vests or has vested after December 31, 2004. If you accept this offer with respect to your eligible options, the exercise price will be amended, and any cash payment made, only with respect to the portion that vests or vested after December 31, 2004.

        Eligible options include only those options that are outstanding, meaning options that have not been cancelled, have not expired and which have not been exercised in full, as of the expiration date of this offer. If you hold an eligible option, it does not mean that all of your options are subject to Section 409A. Accordingly, if you hold more than one option, you may hold both eligible options and options that are not affected by Section 409A.

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Your election form will identify which of your stock options are eligible options for purposes of this offer. If you hold an option that is not listed in your election form, that option is not an eligible option for purposes of this offer.

        In exchange for their eligible options, participants who accept this offer will receive amended options with the applicable revised option exercise price set forth in their election form and a cash payment equal to the difference between the amended exercise price and the original exercise price, multiplied by the number of unexercised shares of Aon common stock subject to such option. The cash payments will be paid on the first regular payroll date in January 2008, and will be subject to applicable tax withholding. The vesting schedule and expiration dates of options amended in this offer will remain the same as in the original option grant. Under United States tax law, however, stock options issued with an exercise price less than fair market value on the date of issuance do not qualify as incentive stock options ("ISOs"). Accordingly, all eligible options (even if you choose not to participate in this offer) and any amended options will be reported by us as nonqualified stock options for United States tax law purposes.

Q8.    How do I participate in this offer?

A8.    Today we are sending you an e-mail message containing your election form, which you must complete, sign and submit electronically through Lotus Notes. If you wish to participate in this offer, you must submit a properly completed and electronically signed election form through Lotus Notes prior to 11:59 p.m., Chicago Time, on September 17, 2007. Only election forms that are properly completed and submitted in this manner through Lotus Notes by the expiration date will be accepted. Election forms that are submitted after the expiration date will not be accepted. Aon intends to confirm the receipt of your election form and/or any withdrawal form by an e-mail message to you within two business days. If you have not received an e-mail confirmation, you should confirm that we have received your election form and/or any withdrawal form. Election forms submitted by any means other than that described above are not permitted without the prior consent of Jeremy Farmer, Senior Vice President and Head of Human Resources.

        If you participate in this offer, you will be required to accept the offer with respect to all of your eligible options. To help you determine your outstanding eligible option grants and give you the information to make an informed decision, your election form will set forth your eligible options, the new exercise price that would apply to each eligible option (if amended) and a description of any cash payment. If you hold an option that is not listed on the election form, the option is not an eligible option.

        This is a one-time offer, and we will strictly enforce the offering period. We reserve the right to reject any election to accept this offer that we determine is not in appropriate form or that we determine is unlawful to accept. Subject to the terms and conditions of this offer, promptly after the expiration of this offer we will accept all eligible options with respect to which a proper election has been made. We will be deemed to have accepted eligible options for you with respect to which valid elections have been made and not properly withdrawn as of the time when we give written notice to the option holders generally of our acceptance of the options for amendment. (See Section 4)

        Your election to participate becomes irrevocable after 11:59 p.m., Chicago Time, on September 17, 2007, unless the offer is extended past that time, in which case your election will become irrevocable after the new expiration date. The only exception is that if we have not accepted your options by 11:59 p.m., Chicago Time on the second business day following the expiration date, you may withdraw your election at any time thereafter until we accept your options.

        We may extend this offer. If we extend this offer, we will issue a press release, e-mail or other communication disclosing the extension no later than 9:00 a.m., Chicago Time, on the next business day following the previously scheduled expiration date.

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Q9.    If I decide to participate in the offer, what will happen to my current eligible options?

A9.    If you elect to participate in the offer, all of your eligible options will be amended on the next business day following the expiration date. The amendment date will be September 18, 2007, unless the offer period is extended. As of that same date, you will become entitled to receive the Option Consideration described below, subject to any applicable vesting and less applicable tax withholding. Please note that, in order to process these option amendments with E*TRADE, your amended options may not be exercisable for up to three business days following the amendment date. (See Section 6)

Q10.    What will I receive in return for my eligible options?

A10.    Each eligible option was granted with an exercise price that has been determined to be less than the fair market value of Aon's common stock on the deemed grant date. Your election form will identify which of your stock options are eligible options for purposes of this offer. If you accept our offer to amend all of your eligible options, all of your eligible options will be amended to increase the original exercise price to the fair market value of a share of our common stock on the deemed grant date (as listed on your election form). The amended option will continue to be subject to the terms and conditions of the Plan under which it was granted, and to an amended option agreement between you and Aon. The vesting schedule and expiration dates of options amended in this offer will remain the same as in the original option grant.

        In addition to an amended option, you will also receive a cash payment equal to the difference between the amended exercise price and the original exercise price, multiplied by the number of shares of Aon common stock subject to the amended unexercised option. Each such cash payment will be paid on the first regular payroll date in January 2008, and will be subject to applicable tax withholding. The total cash payments you may be entitled to receive in connection with your amended options will be listed on your election form.

The examples below illustrate the difference between accepting and declining the offer:

Assumptions

Options subject to 409A treatment:		1,000
Original exercise price:	$	25.00
Amended exercise price:	$	27.00
Market price at exercise:	$	38.00

        In both examples, it is assumed that all Eligible Options are exercised at the market price of $38.00 in 2007.

Example 1:

		Options	Net Value		Pre-Tax Gain
Re-priced value (net value = market value - amended price)		1,000	$	11.00	$	11,000
Cash reimbursement for lost value (net value = amended price - original price)		1,000	$	2.00	$	2,000
	Total Return (before income tax)				$	13,000

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Example 2:

		Options	Net Value		Pre-Tax Gain
Unadjusted gain (net value = market price - original price)		1,000	$	13.00	$	13,000
Federal excise tax 20%					$	2,600
	Total Return* (before income tax)				$	10,400

*

Under 409A provisions, interest and penalties will also apply. Additional state law taxes may also apply, for example in California.

Q11.    When will I receive my cash payments and amended options?

A11.    If you accept this offer, any cash payment owed to you in connection with the amendment of all of your eligible options will be paid to you on the first regular payroll date in January 2008, and will be subject to applicable tax withholding. Promptly following the expiration of the offer, participating employees will receive a "Promise to Make Cash Payment" evidencing your right to receive a cash payment for your options. This payment will not be subject to any vesting conditions or otherwise be subject to forfeiture. (See Section 6)

        If you accept this offer, your eligible options will be amended on the amendment date. The amendment date will be the next business day following the date on which this offer expires. We expect the amendment date will be September 18, 2007. If the expiration date of the offer is delayed, the amendment date will be similarly delayed. Promptly following the expiration of the offer, we will send you an "Amendment to Stock Option Agreements," which you will be asked to promptly sign and return to us. Please note that, in order to process these option amendments with E*TRADE, your amended options may not be exercisable for up to three business days following the amendment date. (See Section 6)

        You will be entitled to receive cash payments regardless of whether you remain employed with Aon on the actual cash payment date (which will be in January 2008).

Q12.    Why do I have to wait until 2008 to receive a cash payment?

A12.    Section 409A permits us to make cash payments to holders of affected options in connection with amendments to options granted at a discount. The amendments to the exercise price of the affected options address the potential adverse tax consequences of Section 409A, and the cash payments are intended to compensate the option holder for the lost benefit of the lower exercise price. However, under the same guidance, we are not permitted to make the cash payment in the same calendar year in which the options are amended. Since we expect the offer to expire (and eligible options to be amended for those who elect to participate) during calendar year 2007, we are required to wait until calendar year 2008 to make any cash payments.

Q13.    Am I required to participate in this offer?

A13.    No. Participation in this offer is completely voluntary.

        However, if you do participate in this offer, you must accept this offer with respect to all of your eligible options. (See Section 2)

If you do not participate in this offer, your eligible options will remain subject to adverse tax consequences under Section 409A. Please also see Question and Answers 5 and 18 for a description of the potential consequences to you if you decide not to participate in the offer and instead keep your current options.

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        We recommend that you consult with your personal tax advisor to help determine if participation in this offer is right for you.

Q14.    What will be the exercise price of my amended options?

A14.    Amended options will have an exercise price per share equal to the fair market value of our common stock on the option's deemed grant date as listed on your election form. (See Section 9)

Q15.    Once my options are accepted, is there anything I must do to receive the cash payments or amended options?

A15.    You do not need to do anything in order to receive your cash payments for eligible options that are accepted as of the offer expiration date. Promptly following the expiration of the offer, participating employees will receive a "Promise to Make Cash Payment" evidencing their right to receive a cash payment for these options. Participating employees will receive cash payments for these options on the first regular payroll date in January 2008, subject to applicable withholding tax. This payment will not be subject to any vesting conditions or otherwise be subject to forfeiture.

        Once the offer has expired and your election has been accepted, your eligible options will be amended. Promptly following the expiration of the offer, we will send you an "Amendment to Stock Option Agreements," which you will be asked to sign and return to us. Your amended options will be amended on the next business day following the expiration date of this offer. Please note that, in order to process the changes with E*TRADE, your amended options may not be exercisable for up to three business days following the amendment date. (See Section 6)

Q16.    When will my amended options vest?

A16.    If your option is amended, it will continue to vest according to the vesting schedule of your original option. Continued vesting is subject to the terms and conditions of your original option and the applicable Plan under which it was granted, including your continued employment with us through each relevant vesting date. (See Section 9)

Q17.    Will the terms and conditions of my amended options be the same as my original options?

A17.    Yes. Except for the exercise price, the terms and conditions of your amended options will remain the same as the original options they replace. (See Section 9)

        Under United States tax law, however, stock options issued with an exercise price less than fair market value on the date of issuance do not qualify as ISOs. Accordingly, all eligible options (even if you choose not to participate in this offer) and any amended options will be reported by us as nonqualified stock options for United States tax law purposes. (See Section 14)

Q18.    What happens to my options if I do not turn in my election form by the expiration date, choose not to participate in this offer or my options are not accepted?

A18.    If we do not receive your election form by the expiration date, you choose not to participate in this offer, or your options are not accepted by us under this offer, your existing options will (1) remain outstanding until they expire by their terms, (2) retain their current exercise price, and (3) retain their current vesting schedule. If you do not participate in this offer, you may be subject to certain adverse tax consequences under Section 409A with respect to the portion of your affected options vesting after December 31, 2004, including income tax at vesting, an additional 20% tax, interest charges and similar state tax penalties, and we will not reimburse you for any such taxes or any related penalties, charges or fees. (See Section 14)

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Q19.    If I hold multiple eligible options, can I choose the options for which I want to accept this offer?

A19.    No. You may only choose to amend all or none of your eligible options.

Q20.    If I choose to participate in this offer, are there circumstances under which my eligible options would be amended and I would not receive a cash payment?

A20.    No. However, if you choose to accept this offer with respect to eligible options that are unvested and if you are no longer an employee of Aon or one of our subsidiaries on the date that the original option would have vested, any amended options you hold will cease to vest and will terminate in accordance with their terms. Your employment with Aon will remain "at-will" regardless of your participation in the offer and can be terminated by you or us at any time, with or without cause or notice. (See Sections 1 and 2)

        Moreover, even if we accept your eligible options, your option will not be amended if we are prohibited from doing so by applicable laws. For example, we could become prohibited from amending options as a result of changes in SEC or NYSE rules. We do not anticipate any such prohibitions at this time. (See Section 13)

Q21.    How will Aon confirm to me that my election form or withdrawal form has been received?

A21.    We intend to confirm the receipt of your election form and/or any withdrawal form by e-mail within two business days. If you have not received an e-mail confirmation within two business days, you should contact the Aon Tender Offer Call Center, at 1-888-295-1814, to confirm that we have received your election form and/or any withdrawal form. The hours of operation of the Aon Tender Offer Call Center are: 8:00 a.m. to 5:00 p.m. Chicago Time.

Q22.    Can I accept this offer with respect to shares of Aon common stock that I previously acquired upon exercise of Aon options?

A22.    No. This offer relates only to eligible options to purchase shares of Aon common stock. You may not accept this offer with respect to any other shares of Aon common stock. (See Section 12)

Q23.    Will my decision to participate in the offer have an impact on my ability to receive options in the future?

A23.    No. Your decision to participate or not participate in the offer will not have any effect on our making future grants of options to purchase common stock, or any other rights to you or anyone else. (See Section 4)

Q24.    Is this a modification of options?

A24.    Yes. This is considered a modification of options. As a result, Aon will record additional stock-based compensation expense based on the incremental fair value of the amended options and the cash payments. (See Section 12)

Q25.    Will the amendment of my option change the tax treatment of my option?

A25.    Yes. First, you should understand that the tax treatment of your option, regardless of whether it is amended in this offer, may be different from the expected tax treatment when it was originally granted. Under United States tax law, stock options issued with an exercise price less than fair market value on the date of issuance do not qualify as ISOs. Accordingly, all eligible options (even if you choose not to participate in this offer) and any amended options will be reported by us as nonqualified stock options for United States tax law purposes. The tax treatment for a nonqualified stock option is

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different than that of an ISO, in that gains on nonqualified stock options are treated as compensation income at the time of exercise and are subject to tax withholding by Aon.

        A holder of nonqualified stock options will generally realize taxable compensation income when he or she exercises the option, in an amount equal to the difference between the exercise price of the option and the fair market value of the shares subject to the option on the date of exercise. If you do not amend your eligible options in this offer, however, because of Section 409A, you may recognize taxable income when the option vests (to the extent it vests after December 31, 2004) rather than when the option is exercised. You may also incur additional excise taxes and interest under Section 409A. For more detailed information, please read the rest of the Offer to Amend, and see Question and Answer 5 and the tax disclosure set forth under the section entitled "Material United States federal income tax consequences." (Section 14)

        We recommend that you read the tax discussion in this Offer to Amend and discuss the personal tax consequences of nonqualified stock options with your financial, legal and/or tax advisors. (See Sections 3 and 14)

Q26.    How does Aon determine whether I have properly accepted this offer?

A26.    We will determine, in our sole discretion, all questions about the validity, form, eligibility (including time of receipt), and acceptance of eligible options. Our determination of these matters will be final and binding on all parties. We reserve the right to reject any election form or any options under this offer that we determine are not in appropriate form or that we determine are unlawful to accept. We will accept all eligible options with respect to which proper elections are made that are not validly withdrawn, subject to the terms of this offer. No elections will be deemed to have been properly made until all defects or irregularities have been cured by you or waived by us. We have no obligation to give notice of any defects or irregularities in any election form, and we will not incur any liability for failure to give any notice. (See Section 4)

Q27.    When will my amended options expire?

A27.    Your amended options, if any, will expire on the same date as the scheduled expiration of your original options or earlier upon your termination of employment or service with Aon, in accordance with the terms of your original options and the applicable Plan under which they were granted. (See Section 9)

Q28.    Will I receive an amended option agreement?

A28.    Yes. All amended options will be subject to an amended option agreement between you and Aon. Promptly following the expiration of the offer, we will send you an "Amendment to Stock Option Agreements," which you will be asked to promptly sign and return to us. (See Sections 6 and 9)

Q29.    Are there any conditions to this offer?

A29.    Yes. Although the implementation of this offer is not conditioned upon it being accepted with respect to a minimum number of eligible options, the completion of this offer is subject to a number of customary conditions that are described in Section 7 of this Offer to Amend. (See Section 7)

Q30.    If you extend the offer, how will you notify me?

A30.    If we extend this offer, we will issue a press release, e-mail or other form of communication disclosing the extension no later than 9:00 a.m., Chicago Time, on the next business day following the previously scheduled expiration date. (See Sections 2 and 15)

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Q31.    How will you notify me if the offer is changed?

A31.    If we change the offer, we will issue a press release, e-mail or other form of communication disclosing the change no later than 9:00 a.m., Chicago Time, on the next business day following the day we change the offer. (See Section 15)

Q32.    Can I change my mind and withdraw from this offer?

A32.    Yes. You may change your mind after you have submitted an election form and withdraw from the offer at any time before the expiration date. If we extend the expiration date, you may withdraw your election with respect all (but not a portion) of your eligible options at any time until the extended offer expires. You may change your mind as many times as you wish, but you will be bound by the last properly submitted election or withdrawal form we receive before the expiration date. If we have not accepted your options by 11:59 p.m. Chicago Time, on the second business day following the expiration date, you may withdraw your election at any time thereafter until we accept your options. (See Section 5)

Q33.    How do I withdraw my election?

A33.    To withdraw your election with respect to all of your eligible options, you must do the following before the expiration date:

1.

Properly complete and sign the accompanying withdrawal form; and



2.

Scan the properly completed withdraw and e-mail as a .pdf file to exec_compensation@aon.com or send the properly completed withdrawal form to Aon by facsimile to 610-834-3390.

Q34.    What if I withdraw my election and then decide again that I want to participate in this offer?

A34.    If you have withdrawn your election to participate and then decide again that you would like to participate in this offer, you may re-elect to participate by submitting a new properly completed election form through Lotus Notes before the expiration date. The new election form must be electronically signed and submitted after the date of your withdrawal form.

        The new election form will supersede all other previously submitted forms. (See Section 5)

Q35.    How should I decide whether or not to accept this offer with respect to all of my eligible options?

A35.    Participating in this offer does carry risk (see "Risks of Participating in the Offer" on page 19 for information regarding some of these risks), and there are no guarantees that participating in this offer and accepting the Option Consideration will be the right choice for every holder of affected options, even considering the potential tax consequences of keeping them (as described in Section 14). The decision to participate in the offer must be your own. We recommend that you consult with your personal tax advisor on the tax consequences of your decision to help determine if participation in this offer is right for you. (See Section 3)

Q36.    What happens to my options if my employment with Aon terminates before the offer expires?

A36.    If your employment with Aon terminates before the offer expires, you will not be eligible to participate in this offer. Accordingly, any options with respect to which you have accepted this offer will be returned to you and will not be amended pursuant to this offer. Options granted under the Plans generally are exercisable, to the extent vested at the date of termination, for the period specified in

17

your award agreement from the date of termination, if the option holder's employment terminated for a reason other than death or disability. (See Sections 6 and 9)

Q37.    What happens to my options if my employment with Aon terminates after the expiration of the offer?

A37.    If you participate in this offer, but your employment with Aon terminates after the offer expires, you will receive the Option Consideration in this offer. All amended options that you receive will be governed by the terms of the applicable Plan under which they were granted. Options granted under the Plans generally are exercisable, to the extent vested at the date of termination, for the period specified in your award agreement from the date of termination, if the option holder's employment terminated for a reason other than death or disability. If an option holder is terminated for "cause" or is otherwise determined by the Organization & Compensation Committee to have engaged in certain acts of misconduct, the option holder's right to exercise options is subject to termination and unexercised options are subject to forfeiture under the Plans. If your employment with Aon terminates after the expiration of the offer but before you receive any cash payment to which you are entitled through this offer, you will still be eligible to receive the cash payment. (See Sections 6 and 9)

Q38.    Does Section 409A impact shares purchased pursuant to the Employee Stock Purchase Plan (ESPP)?

A38.    No. Shares purchased pursuant to the Employee Stock Purchase Plan are not impacted by Section 409A or this offer.

Q39.    Whom can I talk to if I have questions about the offer, or if I need additional copies of the offer documents?

A39.    Today we are sending you an e-mail containing your election form, which you must complete, sign and submit electronically through Lotus Notes. If you need additional copies of the offer documents or the election or withdrawal forms or have questions regarding this offer, you should contact the Aon Tender Offer Call Center, at 1-888-295-1814. The hours of operation of the Aon Tender Offer Call Center are: 8:00 a.m. to 5:00 p.m. Chicago Time.

18

RISKS OF PARTICIPATING IN THE OFFER

Participating in the offer involves a number of risks, including those described below. In addition, the risks described under the heading entitled "Risk Factors" of Item 1A. of our Annual Report on Form 10-K for the year ended December 31, 2006, filed with the SEC highlight the material risks of participating in this offer, and are incorporated herein by reference. You should carefully consider these risks and are encouraged to speak with your financial, legal and/or tax advisors as necessary before deciding to participate in the offer. In addition, we strongly urge you to read the sections and schedule in this Offer to Amend discussing the tax consequences in the United States, as well as the rest of this Offer to Amend, for a more in-depth discussion of the risks that may apply to you before deciding to participate in the offer.

In addition, this offer and our filings with the SEC contain certain statements related to future results, or state our intentions, beliefs and expectations or predictions for the future which are forward-looking statements as that term is defined in the Private Securities Litigation Reform Act of 1995. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results depending on a variety of factors. Potential factors that could impact results include: general economic conditions in different countries in which we do business around the world, changes in global equity and fixed income markets that could affect the return on invested assets, fluctuations in exchange and interest rates that could influence revenue and expense, rating agency actions that could affect our ability to borrow funds, funding of our various pension plans, changes in the competitive environment, our ability to implement restructuring initiatives and other initiatives intended to yield cost savings, our ability to successfully execute strategic options for our Combined Insurance subsidiary, the impact of current, pending and future regulatory and legislative actions that affect our ability to market or sell, and be reimbursed at current levels for, our Sterling subsidiary's Medicare Advantage health plans, changes in commercial property and casualty markets and commercial premium rates that could impact revenues, changes in revenues and earnings due to the elimination of contingent commissions, other uncertainties surrounding a new compensation model, the impact of investigations brought by state attorneys general, state insurance regulators, federal prosecutors, and federal regulators, the impact of class actions and individual lawsuits including client class actions, securities class actions, derivative actions and ERISA class actions, the impact of the analyses of practices relating to stock options, the cost of resolution of other contingent liabilities and loss contingencies, and the difference in ultimate paid claims in our underwriting companies from actuarial estimates.

The following discussion should be read in conjunction with the summary financial information included and incorporated by reference in this Offer to Amend, as well as the information set forth in this Offer to Amend under the caption "Additional information" in Section 17.

Tax-Related Risks

The IRS could change the expected Section 409A tax consequences.

        The tax consequences of Section 409A on your eligible options described in Section 14 of this Offer to Amend are based on current IRS guidance. The IRS may provide additional guidance with respect to Section 409A. It is possible that such guidance could be significantly different from the current guidance. New guidance could impose less onerous tax consequences on your affected options and, as a result, depending on your personal tax situation, it might have been more beneficial to you not to participate in the offer and to have retained your affected options. All option holders should consult with their own personal tax advisor as to the tax consequences of their participation in the Offer.

        Although we have designed this offer in a way that is specifically contemplated by the U.S. Treasury Department and the U.S. Internal Revenue Service to avoid adverse tax treatment under Section 409A, there can be no guarantee of any specific tax treatment with respect to this offer or in the future should the tax laws change again in a manner that would adversely impact your new options. In that event, Aon cannot provide any assurance that an offer similar to this one will be made.

19

Tax-related risks for residents of certain states.

        If you are subject to tax in the United States, you should be aware that certain states have laws similar to Section 409A. Consequently, you may incur additional taxes, penalties and interest charges under state law provisions. For example, California has a provision similar to Section 409A and imposes a 20% tax in addition to applicable income taxes. You should be certain to consult your personal tax advisor to discuss these consequences.

Tax-related risks for tax residents of multiple countries.

        If you are subject to the tax laws in more than one jurisdiction, you should be aware that there may be tax and social security consequences of more than one country that may apply to you. For example, certain eligible U.S. employees may be subject to the tax laws in the United States and to the tax laws in Canada. You should consult your personal tax advisor to discuss these consequences.

20

THE OFFER

1.     Eligibility.

        You are an "eligible employee" only if you are a current employee of Aon Corporation or our subsidiaries (collectively referred to as "Aon," the "Company," "we," "our" or "us") holding eligible options to purchase shares on Aon common stock that are outstanding on the last date on which this offer remains open for acceptance (the "expiration date") and who is subject to taxation in the United States. Our executive officers and Directors, listed on Schedule A to this Offer to Amend, are not eligible employees and not eligible to participate in this offer. In order to participate in the offer, you must hold eligible options. Only persons subject to taxation in the United States are eligible employees.

        Your employment with Aon will remain "at-will" and can be terminated by you or us at any time, with or without cause or notice.

2.     Number of options and amount of Option Consideration; expiration date.

        Subject to the terms and conditions of this offer, we will accept for amendment eligible options that are held by eligible employees and with respect to which proper elections are made, and are not validly withdrawn, before the expiration date.

        An option to purchase common stock is eligible for this offer only if each of the following conditions is met:

•

the option was granted under the Aon Stock Option Plan (as amended, the "Aon Stock Option Plan"), or the Aon Stock Incentive Plan (as amended, the "Aon Stock Incentive Plan" and together with the Aon Stock Option Plan, the "Plans");



•

the option has an exercise price per share that was less than the fair market value per share of Aon common stock underlying the option on the option's measurement date for accounting purposes (the "deemed grant date");



•

the option was unvested as of December 31, 2004 (if a portion of an option was unvested as of December 31, 2004, only the unvested portion is eligible for this offer);



•

the option is outstanding as of the last date on which this offer remains open for acceptance; and



•

the option is held by an eligible employee.

        As noted above, eligible options include only those options that are outstanding, meaning options that have not been cancelled, have not expired and which have not been exercised in full, as of the expiration date of this offer. For example, if a particular option grant expires after commencement of this offer, but before the expiration date, that particular option grant is not eligible for this offer.

        If you choose to accept this offer, you must accept this offer with respect to all of your eligible options. If you have exercised a portion of an eligible option, your election will apply to the portion that remains outstanding and unexercised.

        Each eligible option was granted with an exercise price that has been determined to be less than the fair market value of Aon's common stock on the deemed grant date. If you accept our offer to amend all of your eligible options, each eligible option you hold will be amended to increase the original exercise price to the fair market value of a share of our common stock on the deemed grant date (as listed on your election form). The amended option will continue to be subject to the terms and conditions of the Plan under which it was granted, and to an amended option agreement between you and Aon. In determining the fair market value of our common stock on a given date, we use the

21

closing price of our common stock on the NYSE. Accordingly, the revised option exercise price in your election form reflects the closing price of our common stock on the NYSE on the deemed grant date.

        In addition to an amended option, you will also receive a cash payment equal to the difference between the amended exercise price of Aon common stock and the original exercise price, multiplied by the number of shares of Aon common stock subject such unexercised option. Each such cash payment will be paid on the first regular payroll date in January 2008, and will be subject to applicable tax withholding. The total cash payment you may be entitled to receive in connection with your amended options will be listed on your election form.

        Eligible options include only those options that are outstanding, meaning options that have not been cancelled, have not expired and which have not been exercised in full, as of the expiration date of this offer. If you hold an eligible option, it does not mean that all of your options are subject to Section 409A. Accordingly, if you hold more than one option, you may hold both eligible options and options that are not affected by Section 409A. Your election form will identify which of your stock options are eligible options for purposes of this offer. If you hold an option that is not listed in your election form, that option is not an eligible option for purposes of this offer.

        In exchange for their eligible options, participants who accept this offer will receive amended options with the applicable revised option exercise price shown in their election form, and a cash payment equal to the difference between the amended exercise price and the original exercise price, multiplied by the number of shares of Aon common stock subject to such unexercised option, less applicable tax withholding. The vesting schedule and expiration dates of options amended in this offer will remain the same as in the original option grant.

        Under United States tax law, however, stock options issued with an exercise price less than fair market value on the date of issuance do not qualify as ISOs. Accordingly, all eligible options (even if you choose not to participate in this offer) and any amended options will be reported by us as nonqualified stock options for United States tax law purposes.

        Only those eligible options that are unexercised as of the end of the offer period (expected to be September 17, 2007) are eligible to receive the amended option and cash payment. If you have exercised a portion of an option grant, you may accept this offer with respect to the unexercised portion of such option grant. (See Section 2)

The examples below illustrate the difference between accepting and declining the offer:

Assumptions

Options subject to 409A treatment:		1,000
Original exercise price:	$	25.00
Amended exercise price:	$	27.00
Market price at exercise:	$	38.00

        In both examples, it is assumed that all Eligible Options are exercised at the market price of $38.00 in 2007.

22

Example 1:

		Options	Net Value		Pre-Tax Gain
Re-priced value (net value = market value - amended price)		1,000	$	11.00	$	11,000
Cash reimbursement for lost value (net value = amended price - original price)		1,000	$	2.00	$	2,000
	Total Return (before income tax)				$	13,000

Example 2:

		Options	Net Value		Pre-Tax Gain
Unadjusted gain (net value = market price - original price)		1,000	$	13.00	$	13,000
Federal excise tax 20%					$	2,600
	Total Return* (before income tax)				$	10,400

*

Under 409A provisions, interest and penalties will also apply. Additional state law taxes may also apply, for example in California.

        If you accept this offer with respect to options that are unvested and you are no longer an employee of Aon on the date that the original option would have vested, the amended options you receive pursuant to this offer will not continue to vest.

        All amended options will be subject to the terms of the applicable Plan under which they were granted, and to an amended option agreement between you and Aon. The current form of option agreement under the Aon Stock Incentive Plan is attached as an exhibit to the Schedule TO that has been filed with the SEC in connection with this offer. See Section 9 of this Offer to Amend for a description of the Plans.

        The expiration date for this offer will be 11:59 p.m., Chicago Time, on September 17, 2007, unless we extend the offer. We may, in our sole discretion, extend the offer, in which event the expiration date shall refer to the latest time and date at which the extended offer expires. See Section 15 of this Offer to Amend for a description of our rights to extend, terminate and amend the offer.

3.     Purpose of the offer.

        In connection with our internal review of Aon's stock option granting practices, we have determined that a number of stock options were granted by Aon at a discount from fair market value. Under Section 409A, only certain of these options are subject to adverse tax consequences. We determined that these options were granted with an exercise price per share that was less than the fair market value per share of the common stock underlying the option on the option's deemed grant date, meaning the date when all necessary action to approve the grant had been completed. Unfortunately, tax legislation under the American Jobs Creation Act of 2004 and the related tax regulations ("Section 409A") provide that the portion of options that were granted with an exercise price less than fair market value (i.e., at a discount) and vest after December 31, 2004 will subject the option holder to unfavorable federal tax consequences. If the eligible options are amended as contemplated by this offer, the unfavorable tax consequences, as described in Section 14 of this Offer to Amend, will be eliminated. Because the eligible options may have inherent value as a result of the discounted exercise price, the cash payment(s) with respect to the eligible options were designed to make the option holders whole for the adjustment in option exercise price on the expiration date.

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        Except as otherwise disclosed in this offer or in our SEC filings, we presently have no plans or proposals that relate to or would result in:

•

any extraordinary transaction, such as a merger, reorganization or liquidation involving Aon;



•

any purchase, sale or transfer of a material amount of our assets;



•

any material change in our present dividend rate or policy, or our indebtedness or capitalization;



•

any change in our present Board of Directors or management, including a change in the number or term of Directors or to fill any existing Board of Director vacancies or to change any executive officer's material terms of employment;



•

any other material change in our corporate structure or business;



•

our common stock being delisted from the NYSE;



•

our common stock becoming eligible for termination of registration under the Exchange Act;



•

the suspension of our obligation to file reports under the Exchange Act;



•

the acquisition by any person of an amount of our securities or the disposition of an amount of any of our securities, other than in the ordinary course or pursuant to existing options or rights; or



•

any change in our certificate of incorporation or bylaws, or any actions that may impede the acquisition of control of us by any person.

        Neither we nor our Board of Directors makes any recommendation as to whether you should accept this offer, nor have we authorized any person to make any such recommendation. You should evaluate carefully all of the information in this offer and make your own decision about whether to participate in this offer. We recommend that you consult with your personal tax advisor to help determine if participation in this offer is right for you.

4.     Procedures for electing to participate in this offer.

Proper election to participate in this offer.

        Participation in this offer is voluntary. If you participate in this offer with respect to any option grant, you must accept this offer with respect to all of your eligible options. Today we are sending you an e-mail message containing your election form, which you must complete, sign and submit electronically through Lotus Notes.

If you wish to participate in this offer, you must submit a properly completed and electronically signed election form through Lotus Notes prior to 11:59 p.m., Chicago Time, on September 17, 2007. Only election forms that are properly completed and submitted in this manner through Lotus Notes by the expiration date will be accepted. Election forms that are submitted after the expiration date will not be accepted. Aon intends to confirm the receipt of your election form and/or any withdrawal form by an e-mail message to you within two business days. If you have not received an e-mail confirmation, you should confirm that we have received your election form and/or any withdrawal form. Election forms submitted by any means other than that described above are not permitted without the prior consent of Jeremy Farmer, Senior Vice President and Head of Human Resources.

        If we do not receive your election form before the expiration date, then you will not be able to participate in the offer, and all eligible options you currently hold will remain unchanged at their original price and terms. Even if you choose not to participate in this offer, all eligible options, even those intended to be ISOs, will be reported by us as nonqualified stock options for United States tax law purposes.

24

        Your election to participate becomes irrevocable after 11:59 p.m., Chicago Time, on September 17, 2007 unless the offer is extended past that time, in which case your election will become irrevocable after the new expiration date, except as provided in Section 5 of this Offer to Amend. You may change your mind after you have submitted an election form and withdraw from the offer at any time before the expiration date, as described in Section 5 of this Offer to Amend. You may change your mind as many times as you wish, but you will be bound by the last properly submitted election or withdrawal form we receive before the expiration date.

        This is a one-time offer, and we will strictly enforce the offering period. We reserve the right to reject any election to accept this offer that we determine is not in appropriate form or that we determine is unlawful to accept. Subject to the terms and conditions of this offer, we will accept all eligible options with respect to which proper elections are made promptly after the expiration of this offer.

        Our receipt of your election form is not by itself an acceptance of your options pursuant to this offer. For purposes of this offer, we will be deemed to have accepted options with respect to which proper elections have been made and are not properly withdrawn as of the time when we give written notice to the option holders generally of our acceptance of their options. We may issue this notice of acceptance by press release, e-mail or other methods of communication. We presently expect that options will be amended on September 18, 2007, which is the next business day following the expected expiration date. Please note that, in order to process these option amendments with E*TRADE, your amended options may not be exercisable for up to three business days following the amendment date.

Determination of validity; rejection of options; waiver of defects; no obligation to give notice of defects.

        We will determine, in our sole discretion, all questions as to the validity, form, eligibility (including time of receipt) and acceptance of any options. Our determination of these matters will be final and binding on all parties. We reserve the right to reject any election form or any options with respect to which elections have been made that we determine are not in appropriate form or that we determine are unlawful to accept. We will accept all eligible options with respect to which proper elections are made that are not validly withdrawn, subject to the terms of this offer. We also reserve the right to waive any of the conditions of the offer or any defect or irregularity for any particular option holder, provided that if we grant any such waiver, it will be granted with respect to all option holders and options with respect to which elections have been made. No elections will be deemed to have been properly made until all defects or irregularities have been cured by the option holder or waived by us. Neither we nor any other person is obligated to give notice of any defects or irregularities in elections, nor will anyone incur any liability for failure to give any notice. This is a one-time offer. We will strictly enforce the offering period, subject only to an extension that we may grant in our sole discretion.

Our acceptance constitutes an agreement.

        Your election through the procedures described in this Section 4 constitutes your acceptance of the terms and conditions of this offer. Our acceptance of your options for amendment will constitute a binding agreement between Aon and you upon the terms and subject to the conditions of this offer.

5.     Withdrawal rights and change of election.

If you have previously elected to accept this offer with respect to your eligible options, you may withdraw that election with respect to all of your eligible options only in accordance with the provisions of this section.

        You may change your mind after you have submitted an election form and withdraw from the offer at any time before the expiration date. If we extend the expiration date, you may withdraw your election with respect all (but not a portion) of your eligible options at any time until the extended offer

25

expires. You may change your mind as many times as you wish, but you will be bound by the last properly submitted election or withdrawal form we receive before the expiration date.

        In addition, although we intend to accept all eligible options with respect to which valid elections have been made promptly after the expiration of this offer (and the offer is expected to expire on September 17, 2007), if we have not accepted your options by 11:59 p.m. Chicago Time, on the second business day following the expiration date, you may withdraw your election at any time thereafter until we accept your options.

To validly withdraw your election with respect to all of your eligible options, you must properly complete and sign the accompanying withdrawal form; and scan the properly completed withdraw and e-mail as a .pdf file to exec_compensation@aon.com or send the properly completed withdrawal form to Aon by facsimile to 610-834-3390.

        Only withdrawal forms that are properly completed and submitted and actually received by the expiration date will be accepted. The expiration date will be 11:59 p.m., Chicago Time, on September 17, 2007, unless we extend the offer. Withdrawal forms that are received after the expiration date will not be accepted. Aon intends to confirm the receipt of your withdrawal form by e-mail within two business days. If you have not received an e-mail confirmation, you should confirm that we have received your withdrawal form. Only withdrawal forms that are properly completed and actually received by the expiration date will be accepted. Withdrawal forms submitted by any means other than those described above are not permitted without the prior consent of Jeremy Farmer, Senior Vice President and Head of Human Resources.

        You may not rescind any withdrawal. If you have withdrawn your election to participate and then decide again that you would like to participate in this offer, you may re-elect to participate by submitting a new properly completed election form through Lotus Notes before the expiration date by following the procedures described in Section 4 of this Offer to Amend. This new election form must be properly completed, signed and dated after your original election form and after your withdrawal form.

        Neither we nor any other person is obligated to give you notice of any defects or irregularities in any withdrawal form or any new election form, nor will anyone incur any liability for failure to give any notice. We will determine, in our sole discretion, all questions about the validity, form and eligibility (including time of receipt) of withdrawal forms and new election forms. Our determination of these matters will be final and binding on all parties.

Aon intends to confirm the receipt of your withdrawal form and/or any election form by e-mail within two U.S. business days. If you have not received an e-mail confirmation, you should confirm that we have received your withdrawal form and/or any election form. Only withdrawal forms that are complete, signed and actually received by the expiration date will be accepted. Withdrawal forms may be submitted only by e-mail. Withdrawal forms submitted by any other means, including United States mail (or other post), Federal Express (or similar delivery service) or interoffice mail, are not permitted and will not be accepted.

6.     Acceptance of options for amendment and issuance of cash payments and amended options.

        Upon the terms and conditions of this offer and promptly following the expiration date, we will accept for amendment all eligible options with respect to which proper elections have been made that have not been validly withdrawn before the expiration date. We expect that the expiration date will be September 17, 2007, unless the offer period is extended. Subject to the terms and conditions of this offer, if elections with respect to your eligible options are properly made and accepted by us, these options will be amended as of the amendment date, which is the next business day following the expiration date of this offer. If the expiration date is delayed, the amendment date will be similarly

26

delayed. With respect to all eligible options for which you choose to accept this offer, as of the amendment date, you will become entitled to receive the Option Consideration described below, subject to any applicable vesting conditions. Please note that, in order to process these option amendments with E*TRADE, your amended options may not be exercisable for up to three business days following the amendment date.

        For purposes of the offer, we will be deemed to have accepted eligible options for you with respect to which valid elections have been made and are not properly withdrawn as of the time when we give written notice to the option holders generally of our acceptance of the options for amendment. This notice may be made by press release, e-mail or other method of communication. Subject to our rights to terminate the offer, discussed in Section 15 of this Offer to Amend, we currently expect that we will accept promptly after the expiration date all options with respect to which proper elections have been made that are not validly withdrawn.

        If you elect to participate in this offer, you will be entitled to receive for your eligible options the Option Consideration, which will consist of amended options, subject to any applicable vesting conditions, and (other than for executive officers and Directors and certain other specified officers) cash payments, in each case as described in Section 2 of this Offer to Amend. Amended options will be amended on the amendment date, which is the next business day following the expiration date of this offer. The amendment date will be September 18, 2007, unless the offer period is extended. Promptly following the expiration of the offer, we will send you an "Amendment to Stock Option Agreements," which you will be asked to promptly sign and return to us.

        Any cash payment owed to you for an option with respect to which you have chosen to accept this offer will be paid to you on the first regular payroll date in January 2008, less applicable tax withholding. This payment will not be subject to any vesting conditions or otherwise be subject to forfeiture. Accordingly, you will be entitled to receive your cash payment regardless of whether you remain employed with Aon on the actual cash payment date. Promptly following the expiration of the offer, if applicable, we will send you a "Promise to Make Cash Payment" evidencing your right to receive the cash payment. If you elect to participate in the offer and do not receive a Promise to Make Cash Payment within seven U.S. business days after the expiration date, please contact the Aon Tender Offer Call Center, at 1-888-295-1814.

        If your employment with Aon terminates before the offer expires, you will not be eligible to participate in this offer regardless of whether you had previously submitted an election form. Instead, you will keep all of your eligible options. Accordingly, any options with respect to which you have accepted this offer will be returned to you and will not be amended pursuant to this offer. These options will continue to be governed by the applicable Plan under which they were granted and by the existing option agreements between you and Aon. Options granted under the Plans generally are exercisable, to the extent vested at the date of termination, for the period specified in your award agreement from the date of termination, if the option holder's employment terminated for a reason other than death or disability.

        If you participate in this offer, but your employment with Aon terminates after the offer expires, you will receive the Option Consideration in this offer. All amended options that you receive will be governed by the terms of the applicable Plan under which they were granted. Options granted under the Plans generally are exercisable, to the extent vested at the date of termination, for the period specified in your award agreement from the date of termination, if the option holder's employment terminated for a reason other than death or disability. If an option holder is terminated for "cause" or is otherwise determined by the Organization & Compensation Committee to have engaged in certain acts of misconduct, the option holder's right to exercise options is subject to termination and unexercised options are subject to forfeiture under the Plans. If your employment with Aon terminates

27

after the expiration of the offer but before you receive any cash payment to which you are entitled through this offer, you will still be eligible to receive the cash payment.

        Options that we do not accept for amendment will remain outstanding until they expire by their terms and will retain their current exercise price and current vesting schedule. Please see Section 14 of this Offer to Amend for a description of the tax consequences to you of participating or not participating in this offer.

7.     Conditions of the offer.

        Notwithstanding any other provision of this offer, we will not be required to accept any options for amendment, and we may terminate the offer, or postpone our acceptance and amendment of any options for which elections to amend have been made, in each case, subject to Rule 13e-4(f)(5) under the Exchange Act, if at any time on or after the date this offer begins, and before the expiration date, any of the following events has occurred, or has been determined by us to have occurred:

•

there shall have been threatened in writing or instituted or be pending any action, proceeding or litigation seeking to enjoin, make illegal or delay completion of the offer or otherwise relating in any manner, to the offer;



•

any order, stay, judgment or decree is issued by any court, government, governmental authority or other regulatory or administrative authority and is in effect, or any statute, rule, regulation, governmental order or injunction shall have been proposed, enacted, enforced or deemed applicable to the offer, any of which might restrain, prohibit or delay completion of the offer or impair the contemplated benefits of the offer to us (see Section 3 of this Offer to Amend for a description of the contemplated benefits of the offer to us);



•

there shall have occurred:



•

any general suspension of trading in, or limitation on prices for, our securities on the NYSE,



•

the declaration of a banking moratorium or any suspension of payments in respect of banks in the United States,



•

any limitation, whether or not mandatory, by any governmental, regulatory or administrative agency or authority on, or any event that, in our reasonable judgment, might affect the extension of credit to us by banks or other lending institutions in the United States,



•

in our reasonable judgment, any material adverse change in United States financial markets generally, including, a decline of at least 10% in either the Dow Jones Industrial Average, the NYSE Index or the Standard & Poor's 500 Index from the date of the commencement of the offer,



•

the commencement or continuation of a war or other national or international calamity directly or indirectly involving the United States, which could reasonably be expected to affect materially or adversely, or to delay materially, the completion of the offer, or



•

if any of the situations described above existed at the time of commencement of the offer and that situation, in our reasonable judgment, deteriorates materially after commencement of the offer;



•

a tender or offer, other than this offer by us, for some or all of our shares of outstanding common stock, or a merger, acquisition or other business combination proposal involving us,

28

shall have been proposed, announced or made by another person or entity or shall have been publicly disclosed or we shall have learned that:

•

any person, entity or group has purchased all or substantially all of our assets,



•

any person, entity or "group" within the meaning of Section 13(d)(3) of the Exchange Act acquires more than 5% of our outstanding shares of common stock, other than a person, entity or group which had publicly disclosed such ownership with the SEC prior to the date of commencement of the offer,



•

any such person, entity or group which had publicly disclosed such ownership prior to such date shall acquire additional common stock constituting more than 1% of our outstanding shares,



•

any new group shall have been formed that beneficially owns more than 5% of our outstanding shares of common stock that in our judgment in any such case, and regardless of the circumstances, makes it inadvisable to proceed with the offer or with such acceptance for amendment of eligible options, or



•

any person, entity or group shall have filed a Notification and Report Form under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended, or made a public announcement reflecting an intent to acquire us or any of our subsidiaries or any of the assets or securities of us or any of our subsidiaries;



•

there shall have occurred any change, development, clarification or position taken in generally accepted accounting principles that could or would require us to record for financial reporting purposes compensation expense against our earnings in connection with the offer other than as contemplated as of the commencement date of this offer (as described in Section 12);



•

any change or changes shall have occurred in the business, condition (financial or other), assets, income, operations or stock ownership of Aon that have resulted or may result, in our reasonable judgment, in a material impairment of the contemplated benefits of the offer to us (see Section 3 of this Offer to Amend for a description of the contemplated benefits of the offer to us); or



•

any rules or regulations by any governmental authority, the NYSE, or other regulatory or administrative authority or any national securities exchange have been enacted, enforced or deemed applicable to Aon in a way that might, in our reasonable judgment, restrain, prohibit or delay completion of the offer or impair the contemplated benefits of the offer to us (which we believe would occur only as a result of changes to Section 409A, the regulations or interpretations thereunder or other tax laws that would affect this offer or the eligible options).

        If any of the above events occur, we may:

•

terminate the offer and promptly return all eligible options with respect to which elections have been made to the eligible employees;



•

complete and/or extend the offer and, subject to your withdrawal rights, retain all eligible options with respect to which elections have been made until the extended offer expires;



•

amend the terms of the offer; or



•

waive any unsatisfied condition and, subject to any requirement to extend the period of time during which the offer is open, complete the offer.

        The conditions to this offer are for our benefit. We may assert them in our sole discretion regardless of the circumstances giving rise to them before the expiration date. We may waive any condition, in whole or in part, at any time and from time to time before the expiration date, in our

29

discretion, whether or not we waive any other condition to the offer. Our failure at any time to exercise any of these rights will not be deemed a waiver of such rights, but will be deemed a waiver of our ability to assert the condition that was triggered with respect to the particular circumstances under which we failed to exercise our rights. Any determination we make concerning the events described in this Section 7 will be final and binding upon all persons.

8.     Price range of shares underlying the options.

        The Aon common stock that underlies your stock options is traded on the NYSE under the symbol "AOC." The following table shows, for the periods indicated, the high and low intraday sales price per share of our common stock as reported by the NYSE.

		High		Low
Fiscal Year Ending December 31, 2007
	3rd Quarter (through August 9, 2007)	$	43.68	$	38.65
	2nd Quarter	$	43.93	$	37.67
	1st Quarter	$	39.27	$	34.30
Fiscal Year Ending December 31, 2006
	4th Quarter	$	37.11	$	33.07
	3rd Quarter	$	35.30	$	31.01
	2nd Quarter	$	42.76	$	32.94
	1st Quarter	$	42.16	$	33.45
Fiscal Year Ended December 31, 2005
	4th Quarter	$	37.14	$	30.62
	3rd Quarter	$	32.87	$	24.90
	2nd Quarter	$	26.10	$	20.65
	1st Quarter	$	25.44	$	18.17
Fiscal Year Ended December 31, 2004
	4th Quarter	$	29.05	$	18.15
	3rd Quarter	$	29.44	$	24.46

        On August 9, 2007, the closing sale price of our common stock, as reported by the NYSE, was $41.46 per share.

You should evaluate current market quotes for our common stock, among other factors, before deciding whether or not to accept this offer.

9.     Source and amount of consideration; terms of amended options.

Consideration.

        We will issue cash payments and amended options with respect to which proper elections have been made and accepted for eligible options. Cash payments will be made from Aon's general corporate assets.

        If we receive and accept elections from eligible employees of all options eligible for this offer, subject to the terms and conditions of this offer, we will amend options to purchase a total of 942,426 shares of our common stock, or less than oneof the total shares of our common stock outstanding as of June 30, 2007, and we will make cash payments (out of cash on hand) totaling approximately $3.9 million.

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General terms of amended options.

        If you participate in this offer and we accept your election to amend your eligible options, you will receive the Option Consideration described in Section 2 of this Offer to Amend.

        Each amended option will be amended on the amendment date (expected to be September 18, 2007). All amended options will be subject to an amended stock option agreement between you and Aon.

        The terms and conditions of your amended options will remain the same as the options with respect to which you choose to accept this offer that they replace, except for the exercise price. Under United States tax law, however, stock options issued with an exercise price less than fair market value on the date of issuance do not qualify as ISOs. Accordingly, all eligible options (even if you choose not to participate in this offer) and any amended options will be reported by us as nonqualified stock options for United States tax law purposes.

        The following description summarizes the material terms of our Plans. Our statements in this Offer to Amend concerning the Plans and the amended options are merely summaries and do not purport to be complete. The statements are subject to, and are qualified in their entirety by reference to, the Plans, and the form of stock option award agreement under the Plans, which has been filed as an exhibit to the Schedule TO of which this offer is a part. Please contact the Aon Tender Offer Call Center at 1-888-295-1814 to receive a copy of either of the Plans, and the form of stock option award agreement thereunder. We will promptly furnish you copies of these documents upon request at our expense.

Summary of the Aon Stock Incentive Plan and Aon Stock Option Plan

        Each of the Plans permits the granting of ISOs or nonqualified stock options to Eligible Employees; however, no stock options were permitted to be granted under the Aon Stock Option Plan after the Company adopted the Aon Stock Incentive Plan in 2001. The number of common shares subject to options currently outstanding under each Plan is: (A) under the Aon Stock Incentive Plan, approximately 22.7 million shares; and (B) under the Aon Stock Option Plan, approximately 6.5 million shares. As of June 30, 2007, the number of shares available for future issuance under the Aon Stock Incentive Plan was approximately 5.8 million shares. Each of the Plans is administered by the Organization and Compensation Committee of our Board of Directors (the "Administrator").

        Subject to the terms of the Aon Stock Incentive Plan, the Administrator has the discretion to select the employees who will be granted awards, determine the terms and conditions of such awards, and to construe and interpret the provisions of the Aon Stock Incentive Plan and any outstanding awards thereunder. The Administrator retains similar discretion under the Aon Stock Option Plan with regard to stock awards outstanding thereunder.

Term of options.

        The term of options granted under each of the Plans generally is as stated in the stock option award agreement. All options for which the exercise price is adjusted pursuant to this offer will expire on the same date as the scheduled expiration of the options with respect to which you choose to accept this offer. The amended options may expire earlier upon your termination of employment with Aon, in accordance with the terms of your original stock option award agreement.

Termination of employment before the expiration date of the tender offer program.

        Your employment with Aon will remain "at-will" regardless of your participation in the offer and can be terminated by you or us at any time, with or without cause or notice. If your employment terminates before the expiration date of the tender offer program, you will not be eligible to participate

31

in this offer. Accordingly, any eligible options with respect to which you have accepted this offer will be returned to you and will not be amended pursuant to this offer.

Termination of employment after the expiration date of the tender offer program.

        If you participate in this offer, but your employment by Aon or a successor entity terminates after the expiration date, you will still receive the Option Consideration in this offer. If your employment terminates before you receive any cash payment to which you are entitled through this offer, you will still be eligible to receive that cash payment.

        Options granted under the Plans generally are exercisable, to the extent vested, for a stated period of time following the date of termination if the option holder's employment terminates for a reason other than his or her death or disability. If an option holder is terminated for "cause" or otherwise determined by the Administrator to have engaged in certain acts of misconduct, the option holder's right to exercise options is subject to termination and unexercised options are subject to forfeiture, as set forth in the Plans.

Exercise price.

        The Administrator generally determines the exercise price at the time the option is granted. The exercise price of the amended options will have an exercise price per share equal to the fair market value of our common stock on the deemed grant date, as set forth in your Election Form.

        In connection with our internal review of stock option granting practices, we have determined that a number of stock options were granted at a discount from fair market value. Under Section 409A, only certain of these options are subject to adverse tax consequences. We determined that these options were granted with an exercise price per share that was less than the fair market value per share of the common stock underlying the option on the option's deemed grant date, meaning the date when all necessary action to approve the grant had been completed. In determining the fair market value of our common stock on a given date, we use the average of the high and low trading prices of our common stock as reported by the NYSE. Accordingly, the revised option exercise price in your Election Form reflects the average of the high and low trading prices of our common stock as reported by the NYSE on the deemed grant date.

Your Election Form will identify which of your stock options are eligible options for purposes of this offer.

Vesting and exercise.

        Each stock option award agreement specifies the term of the option and the date on which the option becomes exercisable. The Administrator generally determines the terms of vesting. Any amended option you receive will be subject to the same vesting schedule as the option it replaces, and you will receive vesting credit for any vesting that accrued under the original option. That means that upon the amendment date, your amended options will remain vested to the same extent and will continue to vest at the same rate as the options they replace. Continued vesting is subject to your continued service with us through each relevant vesting date, in accordance with the terms of your original options and the Plan pursuant to which your option was granted.

Adjustments upon certain events.

        Events Occurring before the Amendment Date.    Although we do not currently anticipate a merger or acquisition involving Aon Corporation, if we merge or consolidate with or are acquired by another entity, prior to the expiration of the offer, you may choose to withdraw any options with respect to which you elected to accept this offer and your options will be treated in accordance with the terms of the Plan under which they were granted and with your stock option award agreement. Further, if Aon

32

is acquired prior to the expiration of the offer, we reserve the right to withdraw the offer, in which case your options and your rights under them will remain intact and remain exercisable for the time period set forth in your option agreement and you will receive no amended options, cash payments or other consideration for the options. If Aon Corporation is acquired prior to the expiration of the offer but does not withdraw the offer, we (or the successor entity) will notify you of any material changes to the terms of the offer or the amended options, including any adjustments to the exercise price or number of shares that will be subject to the amended options. Under such circumstances, we expect that the type of security, the number of shares covered by each amended option and the exercise price of each option would be adjusted based on the consideration per share given to holders of options to acquire our common stock that are outstanding at the time of the acquisition. As a result of this adjustment, you may receive options for more or fewer shares of the acquirer's common stock than the number of shares subject to the eligible options with respect to which you accept this offer or than the number you would have received pursuant to an amended option if no acquisition had occurred. You should be aware that these types of transactions could significantly affect our stock price, including potentially substantially increasing the price of our shares. Option holders who decide not to participate in this offer might be able to exercise their options before the effective date of the merger or acquisition and sell their Aon common stock before the effective date.

        Finally, if we are acquired after the expiration date, it is possible that an acquirer could terminate your employment and therefore, to the extent that you have any amended options subject to vesting, any amended options you hold will cease to vest and will terminate in accordance with their terms. Regardless of whether you remain an employee or other service provider on the scheduled payment date, you will still receive any payments to which you are entitled as a result of your participation in this offer.

        Events Occurring after the Amendment Date.    If Aon Corporation is acquired after we accept your eligible options pursuant to this offer, the treatment of your amended options in such a transaction will be governed by the terms of the transaction agreement or the terms of the Plan under which they were granted, and your amended option agreements.

        Each of the Plans provides that the Administrator shall make appropriate adjustments to any outstanding stock option award in the event of changes in capital structure, such as: (a) a stock split, subdivision or combination of shares, (b) a merger or consolidation, (c) any sale of all or substantially all of our assets, or (d) our complete liquidation. The Administrator has the sole discretion to determine the manner and form of such adjustments.

Transferability of options.

        Options granted under the Plans generally may not be sold, pledged, assigned, hypothecated, transferred or disposed of other than by will or by the applicable laws of descent and distribution.

Amendment and termination of the Plans.

        The Board of Directors generally may amend, alter, suspend or terminate either of the Plans at any time and for any reason.

Registration of shares underlying the options.

        All of the shares of Aon common stock issuable upon exercise of amended options have been registered under the United States Securities Act of 1933, as amended (the "Securities Act") on registration statements on Form S-8 filed with the SEC. Unless you are an employee who is considered an affiliate of Aon for purposes of the Securities Act, you will be able to sell the shares issuable upon exercise of your amended options free of any transfer restrictions under applicable United States securities laws.

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United States federal income tax consequences.

You should refer to Section 14 of this Offer to Amend for a discussion of the United States federal income tax consequences of the amended options and the options with respect to which you choose to accept this offer, as well as the consequences of accepting or rejecting this offer. In addition, some states may impose additional penalty taxes. If you are a citizen or resident of the United States, and are also subject to the tax laws of another non-U.S. jurisdiction, you should be aware that there might be other tax consequences that may apply to you. We strongly recommend that you consult with your own advisors to discuss the consequences to you of participating or not participating in this offer.

10.   Information concerning Aon; summary financial information.

General.

        Our principal executive offices are located at 200 East Randolph Street, Chicago, IL 60601, and our telephone number is (312) 381-1000. Questions regarding how to participate in this offer should be directed to the Aon Tender Offer Call Center at 1-888-295-1814.

        We serve our clients through three operating segments via our various subsidiaries worldwide:

•

Risk and Insurance Brokerage Services acts as an advisor and insurance broker, helping clients manage their risks, as well as negotiating and placing insurance risk with insurance carriers through our global distribution network.



•

Consulting provides advice and services to clients for employee benefits, compensation, management consulting, communications, human resource outsourcing, strategic human resource consulting, and financial advisory and litigation consulting.



•

Insurance Underwriting provides specialty insurance products, including accident, health and life insurance. We have ceased writing property and casualty business and have placed our remaining lines of business into runoff.

        Our clients include corporations and businesses, insurance companies, professional organizations, independent agents and brokers, governments, and other entities. We also serve individuals through personal lines, affinity groups, and certain specialty operations.

        Incorporated in 1979, Aon is the parent corporation of long-established and more recently acquired companies. Aon has approximately 43,100 employees and does business in more than 120 countries and sovereignties.

Financial Information.

        The information set forth in Item 8, Consolidated Financial Statements and Supplementary Data, Exhibit 12(a) and Exhibit 12(b) of Aon's Annual Report on Form 10-K for the year ended December 31, 2006, filed with the Securities and Exchange Commission (the "SEC") on March 1, 2007, including all material incorporated by reference therein, is incorporated herein by reference. Item 1, Financial Statements, Exhibit 12(a) and Exhibit 12(b) of Aon's Quarterly Report on Form 10-Q for the quarter ended March 31, 2007, filed with the SEC on May 10, 2007, including all material incorporated by reference therein, is incorporated herein by reference. Item 1, Financial Statements, Exhibit 12(a) and Exhibit 12(b) of Aon's Quarterly Report on Form 10-Q for the quarter ended June 30, 2007, filed with the SEC on August 9, 2007, including all material incorporated by reference therein, is incorporated herein by reference. Aon's Annual Report on 10-K and Quarterly Reports on Form 10-Q can be accessed electronically on the SEC's website at http://www.sec.gov.

        Aon's book value per share was $18.25 as of June 30, 2007.

        The information set forth below is not necessarily indicative of results of future operations, and should be read in conjunction with Item 7, "Management's Discussion and Analysis of Financial

34

Condition and Results of Operations" and the consolidated financial statements and related notes thereto included in Item 8 of Aon's Annual Report on Form 10-K for the year ended December 31, 2006 to fully understand factors that may affect the comparability of the information presented below.

Selected Financial Data (millions except stockholder, employee and per share data)

			2006		2005
					As Restated(2)
Income Statement Data
	Commissions and fees		$	6,677	$	6,466
	Premiums and other			1,918		1,759
	Investment income			359		271
		Total revenue	$	8,954	$	8,496
	Income from continuing operations		$	626	$	568
	Discontinued operations			93		167
	Cumulative effect of change in accounting principle, net of tax (1)			1		—
		Net income	$	720	$	735
Diluted Net Income Per Share
	Continuing operations		$	1.86	$	1.68
	Discontinued operations			0.27		0.49
	Cumulative effect of change in accounting principle (1)			—		—
		Net income	$	2.13	$	2.17
Basic Net Income Per Share
	Continuing operations		$	1.98	$	1.75
	Discontinued operations			0.29		0.52
	Cumulative effect of change in accounting principle (1)			—		—
		Net income	$	2.27	$	2.27
Balance Sheet Data
Assets
	Investments		$	7,575	$	7,058
	Brokerage and consulting receivables			8,707		8,039
	Intangible assets			4,679		4,253
	Other			3,357		8,482
		Total assets	$	24,318	$	27,832
Liabilities and Stockholders' Equity
	Insurance premiums payable		$	9,704	$	9,380
	Policy liabilities			2,849		3,501
	Notes payable			2,243		2,105
	General liabilities			4,304		7,529
		Total liabilities		19,100		22,515
	Stockholders' equity			5,218		5,317
		Total liabilities and stockholders' equity	$	24,318	$	27,832
Common Stock and Other Data
	Dividends paid per share		$	0.60	$	0.60
	Price range			42.76-31.01		37.14-20.65
	At year-end:
		Stockholders' equity per share	$	17.42	$	16.56
	Market price		$	35.34	$	35.95
	Common stockholders			10,013		10,523
	Shares outstanding			299.6		321.2
	Number of employees			43,100		46,600

(1)

Adoption of FASB Statement No. 123(R), "Share-Based Payments," effective January 1, 2006, net of tax.

(2)

See the "Explanatory Note" immediately preceding Part 1, Item 1 and Note 2, "Restatement of Consolidated Financial Statements" to the consolidated financial statements of Aon's Annual Report on Form 10-K for the year ended December 31, 2006.

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11.    Interests of Directors and executive officers; transactions and arrangements concerning the options.

        A list of our Directors and executive officers is attached to this Offer to Amend as Schedule A. None of the members of Aon's Board of Directors or Aon's executive officers may participate in the Offer.

        D. Cameron Findlay, Executive Vice President and General Counsel of Aon, exercised options to purchase an aggregate of 67,466 shares of Aon common stock on August 3, 2007 and August 6, 2007.

12.   Status of options amended by us in the offer; accounting consequences of the offer.

        Eligible options accepted by us pursuant to this Offer will be amended under the Aon Stock Incentive Plan and will have no effect on the number of options eligible for future grants under the Aon Stock Incentive Plan.

        The Offer is considered a modification of options and as a result, Aon will record additional stock-based compensation expense.

13.   Legal matters; regulatory approvals.

        We are not aware of any license or regulatory permit that appears to be material to our business that might be adversely affected by our acceptance of options for amendment and issuance of amended options as contemplated by the offer, or of any approval or other action by any government or governmental, administrative or regulatory authority or agency or any NYSE listing requirements that would be required for the acquisition or ownership of our options as contemplated herein. Should any additional approval or other action be required, we presently contemplate that we will seek such approval or take such other action. We cannot assure you that any such approval or other action, if needed, could be obtained or what the conditions imposed in connection with such approvals would entail or whether the failure to obtain any such approval or other action would result in adverse consequences to our business. Our obligation under the offer to accept elections with respect to eligible options and to issue amended options is subject to the conditions described in Section 7 of this Offer to Amend.

        If we are prohibited by applicable laws or regulations from granting amended options on the amendment date, we will not grant any amended options. We are unaware of any such prohibition at this time, and we will use reasonable efforts to effect the grant, but if the grant is prohibited on the amendment date we will not grant any amended options.

14.   Material United States federal income tax consequences.

If You Participate in the Offer to Amend.

        As a result of participation in this offer, you may avoid potentially adverse tax consequences under Section 409A associated with your eligible options. Please read this section carefully, as well as the following section summarizing the potential federal tax consequences to you if you decide not to participate in this offer and to keep your current options.

        The following is a summary of the material United States federal income tax consequences of participating in the offer for those employees subject to United States federal income tax. This discussion is based on the Internal Revenue Code, its legislative history, treasury regulations thereunder and administrative and judicial interpretations as of the date of this Offer to Amend, all of which are subject to change, possibly on a retroactive basis. The federal tax laws may change and the federal, state and local tax consequences for each employee will depend upon that employee's individual circumstances. This summary does not discuss all of the tax consequences that may be relevant to you in light of your particular circumstances, nor is it intended to be applicable in all respects to all

36

categories of option holders. We strongly recommend that you consult with your own tax advisor to discuss the consequences to you of this transaction. If you are subject to taxation in the United States, and are also subject to the tax laws of another country, you should be aware that there might be other tax consequences that may apply to you. In addition, some states, including California, may impose additional penalty taxes on the amounts subject to the 20% federal penalty tax.

Cash payments.

        Any cash payments you receive as part of your Option Consideration under this offer will be taxable to you as compensation income. We generally will be entitled to a deduction equal to the amount of compensation income taxable to you. If you were an employee at the time the options with respect to which you accepted this offer were granted, any income recognized upon your receipt of a cash payment will constitute wages for which withholding will be required.

Amended options.

        If you are an option holder who chooses to accept this offer with respect to outstanding eligible options, you should not be required to recognize income for United States federal income tax purposes at the time of the acceptance and amendment of such options. We believe that the acceptance and amendment of options will be treated as a non-taxable exchange for United States federal income tax purposes.

        Under United States tax law, stock options issued with an exercise price less than fair market value on the date of issuance do not qualify as ISOs. Accordingly, all eligible options (even if you choose not to participate in this offer) and any amended options will be reported by us as nonqualified stock options for United States tax law purposes. Under current law, an option holder generally will not realize taxable income upon the grant of a nonqualified stock option. However, when an option holder exercises the option, the difference between the exercise price of the option and the fair market value of the shares subject to the option on the date of exercise will be compensation income taxable to the option holder. The tax treatment for a nonqualified stock option is different than that of an ISO, in that gains on nonqualified stock options are treated as compensation income at the time of exercise and are subject to income tax withholding by Aon. As a result of Section 409A, however, nonqualified stock options granted with an exercise price below the fair market value of the underlying stock may be taxable to a participant upon vesting, before he or she exercises an award. The revised exercise price of your amended option will avoid that earlier taxation (and related penalties described below).

        We generally will be entitled to a deduction equal to the amount of compensation income taxable to the option holder if we comply with applicable reporting requirements.

        Upon disposition of shares obtained upon exercise of an option, any gain or loss is treated as capital gain or loss. If you were an employee at the time of the grant of the option, any income recognized upon exercise of a nonqualified stock option generally will constitute wages for which withholding will be required.

        We recommend that you consult with your own tax advisor with respect to the federal, state and local tax consequences of participating in the offer.

If You Do Not Participate in the Offer to Amend.

        The following is a summary of the material United States federal income tax consequences of declining to participate in the offer for those employees subject to United States federal income tax. This discussion is based on the Internal Revenue Code, its legislative history, treasury regulations thereunder and administrative and judicial interpretations as of the date of this Offer to Amend, all of which are subject to change, possibly on a retroactive basis. The federal tax laws may change and the federal, state and local tax consequences for each employee will depend upon that employee's

37

individual circumstances. This summary does not discuss all of the tax consequences that may be relevant to you in light of your particular circumstances, nor is it intended to be applicable in all respects to all categories of option holders. If you are subject to taxation in the United States, and are also subject to the tax laws of another country, you should be aware that there might be other tax and social security consequences that may apply to you. We strongly recommend that you consult with your own advisors to discuss the consequences to you of this transaction.

        Your decision not to accept this offer with respect to your eligible options could result in potentially adverse tax consequences to you. Please read this section carefully and talk with your own tax advisor regarding your decision to participate in this offer.

        If you decide not to participate in this offer, you may be subject to adverse tax consequences associated with your eligible options. Section 409A of the Internal Revenue Code and the related tax regulations under the American Jobs Creation Act of 2004 provide that stock options issued with an exercise price less than the fair market value of the underlying stock on the date of grant (i.e., granted at a discount) must have fixed exercise dates to avoid early income recognition and an additional 20% tax. The eligible options were granted at a discount and holders of such options may be subject to income tax at vesting and an additional 20% tax, as well as be liable for certain interest penalties.

        None of the eligible options have fixed exercise dates and therefore they would subject the option holders to income tax at vesting (i.e., before the options are exercised) and would subject the option holders to an additional 20% tax. It is not entirely certain how such tax would be calculated, but we think it is likely that the spread (that is, the difference between the fair market value of the shares on the date of vesting and the exercise price of such shares) will be includable as income when the option vests and a 20% tax will be assessed on the spread. Additionally, it is possible that during each subsequent tax year (until the option is exercised), the increase in value of the underlying stock will be taxed. These consequences would apply to that portion of any eligible option that vests or vested after December 31, 2004. Any portion of an option that vested before that date is not affected by Section 409A.

        As noted above, whether or not you choose to participate in this offer, all affected options, even those intended to be ISOs when originally granted, will be reported by us as nonqualified stock options for United States tax law purposes.

15.   Extension of offer; termination; amendment.

        We reserve the right, in our sole discretion, at any time and regardless of whether or not any event listed in Section 7 of this Offer to Amend has occurred or is deemed by us to have occurred, to extend the period of time during which the offer is open and delay the acceptance for amendment of any options. If we elect to extend the period of time during which this offer is open, we will notify you of the extension and delay, as described below. If we extend the expiration date, we will also extend your right to withdraw elections with respect to eligible options until such extended expiration date. In the case of an extension, we will issue a press release, e-mail or other form of communication no later than 9:00 a.m., Chicago Time, on the next U.S. business day after the previously scheduled expiration date.

        We also reserve the right, in our reasonable judgment, before the expiration date to terminate or amend the offer and to postpone the expiration of the offer (resulting in a delay of our acceptance and amendment of any options with respect to which elections have been made) if any of the events listed in Section 7 of this Offer to Amend occurs, by giving written notice of the termination or postponement to you or by making a public announcement of the termination. Our reservation of the right to delay our acceptance and amendment of options with respect to which elections have been made is limited by Rule 13e-4(f)(5) under the Exchange Act, which requires that we must pay the consideration offered or return the options promptly after termination or withdrawal of an offer like this.

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        Subject to compliance with applicable law, we further reserve the right, before the expiration date, in our sole discretion, and regardless of whether any event listed in Section 7 of this Offer to Amend has occurred or is deemed by us to have occurred, to amend the offer in any respect, including by decreasing or increasing the consideration offered in this offer to option holders or by decreasing or increasing the number of options being sought in this offer. As a reminder, if a particular option grant expires after commencement of this offer, but before amendment under the offer, that particular option grant is not eligible for amendment. Therefore, if we extend the offer for any reason and if a particular option with respect to which an election to accept the offer was made before the originally scheduled expiration of the offer expires after such originally scheduled expiration date but before the actual amendment date under the extended offer, that option would not be eligible for amendment.

        The minimum period during which the offer will remain open following material changes in the terms of the offer or in the information concerning the offer, other than a change in the consideration being offered by us or a change in amount of existing options sought, will depend on the facts and circumstances of such change, including the relative materiality of the terms or information changes. If we modify the number of eligible options being sought in this offer or the consideration being offered by us for the eligible options in this offer, the offer will remain open for at least ten business days from the date of notice of such modification. If any term of the offer is amended in a manner that we determine constitutes a material change adversely affecting any holder of eligible options, we will promptly disclose the amendments in a manner reasonably calculated to inform holders of eligible options of such amendment, and we will extend the offer's period so that at least five business days, or such longer period as may be required by the tender offer rules, remain after such change.

16.   Fees and expenses.

        We will not pay any fees or commissions to any broker, dealer or other person for soliciting elections with respect to this offer.

17.   Additional information.

        This Offer to Amend is part of a Tender Offer Statement on Schedule TO that we have filed with the SEC. This Offer to Amend does not contain all of the information contained in the Schedule TO and the exhibits to the Schedule TO. We recommend that you review the Schedule TO, including its exhibits, and the following materials that we have filed with the SEC before making a decision on whether to elect to accept this offer with respect to your options:

1.

Our annual report on Form 10-K for our fiscal year ended December 31, 2006, filed with the SEC on March 1, 2007;



2.

Our definitive proxy statement on Schedule 14A for our 2007 annual meeting of shareholders, filed with the SEC on April 11, 2007;



3.

Our quarterly report on Form 10-Q for our fiscal quarter ended March 31, 2007, filed with the SEC on May 10, 2007;



4.

Our quarterly report on Form 10-Q for our fiscal quarter ended June 30, 2007, filed with the SEC on August 9, 2007; and



5.

Our current reports on Form 8-K dated July 31, 2007 (only with respect to Item 8.01), July 17, 2007, May 21, 2007, March 15, 2007, February 26, 2007 and January 29, 2007.

        The SEC file number for these filings is 001-07933. These filings, our other annual, quarterly and current reports, our proxy statements and our other SEC filings may be examined, and copies may be obtained, at the SEC's public reference room at 100 F Street N.E., Washington, D.C. 20549. You may obtain information on the operation of the public reference room by calling the SEC at

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1-800-SEC-0330. Our SEC filings are also available to the public on the SEC's Internet site at www.sec.gov.

        Each person to whom a copy of this Offer to Amend is delivered may obtain a copy of any or all of the documents to which we have referred you, other than exhibits to such documents, unless such exhibits are specifically incorporated by reference into such documents, at no cost, by writing to us at Aon Corporation, 200 East Randolph Street, Chicago, IL 60601, Attention: Investor Relations, or telephoning (312) 381-1000.

        As you read the documents listed above, you may find some inconsistencies in information from one document to another. If you find inconsistencies between the documents, or between a document and this Offer to Amend, you should rely on the statements made in the most recent document.

        The information about us contained in this Offer to Amend should be read together with the information contained in the documents to which we have referred you, in making your decision as to whether or not to participate in this offer.

18.   Miscellaneous.

        We are not aware of any jurisdiction where the making of the offer is not in compliance with applicable law. If we become aware of any jurisdiction where the making of the offer is not in compliance with any valid applicable law, we will make a good faith effort to comply with such law. If, after such good faith effort, we cannot comply with such law, the offer will not be made to, nor will options be accepted from the option holders residing in such jurisdiction.

We have not authorized any person to make any recommendation on our behalf as to whether you should elect to accept this offer with respect to your options. You should rely only on the information in this document or documents to which we have referred you. We have not authorized anyone to give you any information or to make any representations in connection with the offer other than the information and representations contained in this Offer to Amend the Exercise Price of Certain Options and in the related offer documents. If anyone makes any recommendation or representation to you or gives you any information, you must not rely upon that recommendation, representation or information as having been authorized by us.

Aon Corporation
August 15, 2007

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SCHEDULE A

INFORMATION CONCERNING THE DIRECTORS AND
EXECUTIVE OFFICERS OF AON CORPORATION

        The directors and executive officers of Aon Corporation are set forth in the following table:

Name	Position and Offices Held
Gregory C. Case	President and Chief Executive Officer
Patrick G. Ryan	Executive Chairman
Edgar D. Jannotta	Director
Jan Kalff	Director
Lester B. Knight	Director
J. Michael Losh	Director
R. Eden Martin	Director
Andrew J. McKenna	Director
Robert S. Morrison	Director
Richard B. Myers	Director
Richard C. Notebaert	Director
John W. Rogers, Jr.	Director
Gloria Santona	Director
Carolyn Y. Woo	Director
Diane Aigotti	Senior Vice President, Chief Risk Officer and Treasurer
Andrew M. Appel	Chief Executive Officer, Aon Consulting Worldwide, Inc.
Gregory J. Besio	Senior Vice President and Head of Global Strategy
David P. Bolger	Executive Vice President, Chief Financial Officer and Chief Administrative Officer
Ted T. Devine	Executive Vice President
Jeremy G.O. Farmer	Senior Vice President and Head of Human Resources
D. Cameron Findlay	Executive Vice President and General Counsel
Bernard Fung	Chief Executive Officer, Aon Asia Pacific
Peter Harmer	Chief Executive Officer, United Kingdom
Roelof Hendriks	Chief Executive Officer, Europe, Middle East and Africa
Daniel F. Hunger	Senior Vice President and Controller
Stephen P. McGill	Chief Executive Officer, Aon Risk Services Americas
Michael D. O'Halleran	Senior Executive Vice President
Richard M. Ravin	Chairman—Combined Insurance Company of America
Douglas R. Wendt	Chief Executive Officer—Combined Insurance Company of America

        None of the members of Aon's Board of Directors or Aon's executive officers may participate in this offer.

        The business address of each director and executive officer is c/o Aon Corporation, 200 East Randolph Street, Chicago, IL 60601, (312) 381-1000, and the telephone number of each director and executive officer is (312) 381-1000.

A-1

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AON CORPORATION
OFFER TO AMEND THE EXERCISE PRICE OF CERTAIN OPTIONS
IMPORTANT—STEPS YOU MUST TAKE TO PARTICIPATE
Offer to Amend the Exercise Price of Certain Options dated August 15, 2007.
TABLE OF CONTENTS
SUMMARY TERM SHEET AND QUESTIONS AND ANSWERS
RISKS OF PARTICIPATING IN THE OFFER
THE OFFER
SCHEDULE A INFORMATION CONCERNING THE DIRECTORS AND EXECUTIVE OFFICERS OF AON CORPORATION

EX-99.(A)(1)(C)

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Exhibit (a)(1)(c)

AON

To:	Employees Eligible to Participate in Offer to Amend Certain Outstanding Options
From:	Jeremy Farmer, Senior Vice President and Head of Human Resources
Date:	August 15, 2007
Re:	Important Information Concerning Your Aon Stock Options

        As indicated in the e-mail sent to you on August 10, 2007, Aon recently performed a voluntary review of its stock option granting practices. It was determined that, as a result of this review, one or more of the stock options you hold may have been granted at a "discounted" exercise price (i.e., the exercise price is less than the fair market value of the stock on the deemed grant date). Unless you take corrective action, all or a portion of the affected option grants could subject you to adverse personal tax consequences.

        Section 409A of the U.S. Internal Revenue Code imposes adverse personal tax consequences on discounted stock options that vest after December 31, 2004. These tax consequences include income tax at vesting (as opposed to upon exercise) and an additional 20% tax on the gain and interest charges. These tax consequences include subjecting your entire gain on the exercise of the discounted option to taxation.

        As discussed in the prior e-mail, Aon does not intend for you to bear the burden of these adverse tax consequences. Accordingly, we are offering you the opportunity to participate in a special option amendment program with respect to the affected stock option grants you hold. If you participate in the program, all of your affected option grants will be amended so that they no longer will subject you to the adverse personal tax consequences and you will be given the difference in price in a cash payment.

        The Offer to Amend the Exercise Price of Certain Options, which was filed today with the Securities and Exchange Commission, is attached to this email. This document contains specific details about the program (called a "tender offer"), including Frequently Asked Questions (beginning on Page 6), which we encourage you to review. You will receive a separate email containing a personalized election form which describes the details of your affected options.

You may only elect to amend the eligible portion of all or none of your eligible options. If you choose not to participate in the tender offer, you will likely be subject to the adverse personal tax consequences described above and we will not reimburse you for any such taxes or any related penalties, charges or fees. You will also not be entitled to the cash payment.

If you wish to participate in this offer, you must submit a properly completed and electronically signed election form through Lotus Notes prior to 11:59 p.m., Chicago Time, on September 17, 2007.

        To help (i) explain the potential adverse personal tax consequences, (ii) explain how Aon has addressed the situation and the choices you have, and (iii) answer any other questions you may have, a webcast will be available to you. You can view this webcast at any time by following the instructions below to access the presentation:

1.

Proceed to Aon's site on Webex—https://aon.webex.com

2.

Click "Recorded Sessions" on the left side bar under "Attend a Session"



3.

Click "Presentation on Aon's Tender Offer Program"



4.

Click the View button to play the webcast

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5.

Enter Recording Password 6N32C

6.

Click OK

7.

Enter your name and internet email address (for example, John_Smith@aon.com)



8.

Click Register

        The webcast will start to play.

        Please note that your computer must support sound in order to view the webcast. If you experience difficulty with the audio portion of the webcast, please contact the Help Desk at +1.866.AON.HELP.

Questions

        If you have any questions about the information contained in this memo, or your participation in the tender offer, please feel free to contact the Aon Tender Offer Call Center, at 1-888-295-1814. The hours of operation of the Aon Tender Offer Call Center are 8:00 a.m. to 5:00 p.m. Chicago Time.

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Exhibit (a)(1)(c)

EX-99.(A)(1)(D)

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Exhibit (a)(1)(d)

AON CORPORATION
OFFER TO AMEND THE EXERCISE PRICE OF CERTAIN OPTIONS
ELECTION FORM

[Text of election form follows:]

Important: Please read this Election Form in its entirety and read the Instructions to this Election Form before
submitting this Election Form.

        I,                        , hereby make the following election with respect to my Eligible Options in the Offer made by Aon Corporation ("Aon") pursuant to the Offer to Amend the Exercise Price of Certain Options dated August 15, 2007 (the "Offer to Amend"). Capitalized terms not otherwise defined in this Election Form shall have the meaning set forth in the Offer to Amend.

        Subject to the foregoing, I hereby elect to amend or not amend all of my Eligible Options as identified in the table below:

Eligible Option (Grant Number)	Original Date of Grant	Original Option Exercise Price	Shares Subject to Eligible Portion	Corrected Exercise Price for Eligible Portion	Cash Payment (Aggregate Price Differential)
		$		$	$
		$		$	$

Amend all Eligible Options?                              o Yes                              o No

        I hereby agree that, unless I revoke my election before 11:59 p.m., Chicago time, on September 17, 2007 (or a later expiration date if Aon extends the Offer), my election will be irrevocable, and if accepted by Aon, this Election Form shall operate to amend all of my Eligible Options as outlined above, subject to the terms and conditions described in the Offer to Amend.

        I hereby acknowledge that I may change the terms of my election by submitting a new Election Form via Lotus Notes. Submissions made by any other means, including hand delivery, facsimile, inter-office mail or U.S. mail (or other postal service) will not be accepted. Any change of election received after the expiration time will be void and of no effect.

        I agree that my decision to amend or not amend my Eligible Options in the Offer is entirely voluntary and is subject to the terms of the Offer. I further understand and agree I am not required to tender my Eligible Options. I understand that with respect to my Eligible Options, I must elect to tender all or none of the Eligible Options for amendment of all Eligible Options. I further understand and agree that if I fail to make an election with respect to my Eligible Options, then I will be deemed to have made an election not to amend my Eligible Options.

        I acknowledge and agree that neither the ability to participate in the Offer nor actual participation in the Offer shall be construed as a right to continued employment with Aon (except on an at-will basis, unless otherwise required by local law). I agree that, except as set forth in the Offer to Amend, Aon has made no representations or warranties to me regarding this Offer or the future pricing of Aon stock, and that my participation in this Offer is at my own discretion.

        If my service with Aon terminates prior to the expiration time, I understand that I will no longer be eligible to participate in the Offer and any election I have made to amend my Eligible Options will be of no further force and effect.

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        I agree that to ensure timely payment of the cash payment, I will provide Aon with any changes in my mailing address and e-mail address prior to the payment of the cash payment.

        I hereby acknowledge and agree that neither Aon nor any of its respective employees or agents, has made any recommendation to me as to whether or not I should accept this Offer to Amend the Exercise Price of Certain Options and that I am not relying on any information provided or representation made by Aon or any of its employees or agents in accepting or rejecting this Offer, other than any information contained in the Offer to Amend. I acknowledge that I have been afforded the opportunity to consult with my own investment, legal and tax advisors before making this election and that I have knowingly done so or knowingly declined to do so.

        I understand that I should print and keep a copy of this completed Election Form as well as a copy of the "delivery receipt" from Lotus Notes that I receive upon submitting the Election Form.

        I understand that I will receive an email confirmation at my Aon email address (as listed below) within two business days after the date on which Aon receives this Election Form. In addition, within two business days after the expiration time, I will receive via email at my Aon email address a final confirmation statement that confirms the last election that I made for my Eligible Options as of the expiration time. I agree that I will print and keep a copy of all election confirmations that I receive. In the event I do not receive these election confirmations in the time frames described above, I understand that it is my responsibility to send my printed copies of this Election Form, the email delivery receipt and any election confirmations that I did receive to exec_compensation@aon.com to evidence proper and timely submission of my Election Form.

        I agree that Aon will not be liable for any costs, taxes, loss or damage that I may incur through my election to participate in or to decline this offer.

Employee Signature

Employee Name (Please print) Company E-mail Address Date and Time

If you wish to participate in this offer, you must submit a properly completed and electronically signed
election form through Lotus Notes prior to 11:59 p.m., Chicago Time, on September 17, 2007.

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AON CORPORATION

OFFER TO AMEND THE EXERCISE PRICE OF CERTAIN OPTIONS

INSTRUCTIONS TO THE ELECTION FORM

FORMING PART OF THE TERMS AND CONDITIONS OF THE OFFER

1.     Delivery of Election Form.

        Today we are sending you an e-mail message containing your election form, which you must complete, electronically sign and submit electronically through Lotus Notes. If you wish to participate in this offer, you must submit a properly completed and electronically signed election form through Lotus Notes prior to 11:59 p.m., Chicago Time, on September 17, 2007. Only election forms that are properly completed and submitted in this manner through Lotus Notes by the expiration date will be accepted. Election forms that are submitted after the expiration date will not be accepted. Aon intends to confirm the receipt of your election form and/or any withdrawal form by an e-mail message to you within two business days. If you have not received an e-mail confirmation, you should confirm that we have received your election form and/or any withdrawal form. Election forms submitted by any means other than that described above are not permitted without the prior consent of Jeremy Farmer, Senior Vice President and Head of Human Resources.

        Our receipt of your election form is not by itself an acceptance of your eligible options for amendment. For purposes of the offer, we will be deemed to have accepted eligible options for exchange with respect to which valid elections have been made and not properly withdrawn when we give oral or written notice to the option holders generally of our acceptance such options, which notice may be made by press release, e-mail or other method of communication.

        Aon will not accept any alternative, conditional or contingent elections. Although it is our intent to send you an e-mail confirmation of receipt of this election form, by electronically signing this election form, you waive any right to receive any notice of the receipt of the election with respect to your options, except as provided for in the Offer to Amend. Any confirmation of receipt sent to you will merely be a notification that Aon has received your election form and does not mean that your options have been accepted or amended. Options that are accepted will be amended on the next business day following the expiration of the offer. If the offer expires as expected on September 17, 2007, then the options will be amended as of September 18, 2007.

2.     Withdrawal

        If you have previously elected to accept this offer with respect to your eligible options, you may withdraw that election at any time before the expiration date, which is expected to be 11:59 p.m., Chicago Time, on September 17, 2007. If we extend the offer, you may withdraw your options at any time until the extended expiration date.

        In addition, although we intend to accept all eligible options with respect to which valid elections have been made promptly after the expiration of this offer (and the offer is expected to expire on September 17, 2007), if we have not accepted your options by 11:59 p.m. Chicago Time, on the second business day following the expiration date, you may withdraw your election at any time thereafter until we accept your options.

To validly withdraw your election with respect to all of your eligible options, you must properly complete and sign the accompanying withdrawal form; and scan the properly completed withdraw and e-mail as a .pdf file to exec_compensation@aon.com or send the properly completed withdrawal form to Aon by facsimile to 610-834-3390.

        Only withdrawal forms that are properly completed and submitted and actually received by the expiration date will be accepted. The expiration date will be 11:59 p.m., Chicago Time, on September 17,

3

2007, unless we extend the offer. Withdrawal forms that are received after the expiration date will not be accepted. Aon intends to confirm the receipt of your withdrawal form by e-mail within two business days. If you have not received an e-mail confirmation, you should confirm that we have received your withdrawal form. Only withdrawal forms that are properly completed and actually received by the expiration date will be accepted. Withdrawal forms submitted by any means other than those described above are not permitted without the prior consent of Jeremy Farmer, Senior Vice President and Head of Human Resources.

3.     Elections.

        If you intend to accept the offer, you must accept the offer with respect to all of your eligible options. You may not choose to accept the offer with respect to only a portion of your eligible options. If you have exercised a portion of an eligible option grant, your election will apply to the portion that remains outstanding and unexercised.

4.     Requests for Assistance or Additional Copies.

        Today we are sending you an e-mail containing your election form, which you must complete, electronically sign and submit electronically through Lotus Notes. If you need additional copies of the offer documents or the election or withdrawal forms or have questions regarding this offer, you should contact the Aon Tender Offer Call Center, at 1-888-295-1814. The hours of operation of the Aon Tender Offer Call Center are: 8:00 a.m. to 5:00 p.m. Chicago Time.

5.     Irregularities.

        We will determine, in our discretion, all questions as to the validity, form, eligibility, including time of receipt, and acceptance of any eligible options. Our determination of these matters will be final and binding on all parties. We reserve the right to reject any election form that we determine are not in appropriate form or that we determine are unlawful to accept. We will accept all eligible options with respect to which proper elections are made that are not validly withdrawn, subject to the terms of the offer. We also reserve the right to waive any of the conditions of the offer or any defect or irregularity in any election, provided that if we grant any such waiver, it will be granted with respect to all option holders and options with respect to which elections have been made. No elections will be deemed to have been properly made until all defects or irregularities have been cured by the option holder or waived by us. Neither we nor any other person is obligated to give notice of any defects or irregularities in elections, nor will anyone incur any liability for failure to give any notice. This is a one-time offer. We will strictly enforce the offering period, subject only to an extension that we may grant in our sole discretion.

Important: The election form together with all other required documents must be received by Aon on or before the expiration date.

6.     Additional Documents to Read.

        You should be sure to read the Offer to Amend, all documents referenced therein (including the election form describing your eligible options and any potential cash payments), and the related memorandum from Jeremy Farmer, Senior Vice President and Head of Human Resources, dated August 15, 2007, before deciding to participate in the offer.

7.     Important Tax Information.

        You should refer to Section 14 of the Offer to Amend, which contains important U.S. federal income tax information. We also recommend that you consult with your personal tax or financial advisor regarding Section 409A and state income tax considerations before deciding whether or not to participate in this offer.

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Exhibit (a)(1)(d)

EX-99.(A)(1)(E)

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Exhibit (a)(1)(e)

COMPLETE AND RETURN THIS FORM ONLY IF YOU HAVE CHANGED YOUR MIND
AND YOU DO NOT WANT TO AMEND ALL OF YOUR ELIGIBLE OPTIONS

AON CORPORATION
OFFER TO AMEND THE EXERCISE PRICE OF CERTAIN OPTIONS
WITHDRAWAL FORM

        Before signing this withdrawal form, please make sure you have received, read and understand the documents that make up this offer, including (1) the Offer to Amend the Exercise Price of Certain Options (the "Offer to Amend"), dated August 15, 2007; (2) the related memorandum from Jeremy Farmer, Senior Vice President and Head of Human Resources, dated August 15, 2007; (3) the election form; and (4) this withdrawal form. You signed and returned the election form, in which you elected to ACCEPT Aon's offer to amend all of your eligible options. You should submit this form only if you now wish to change that election and REJECT Aon's offer with respect to all of your eligible options.

        If you would like to withdraw your election to accept the offer with respect to all of your eligible options, please check the appropriate box below and complete and sign this withdrawal form. Please be sure to follow the instructions which are attached.

        You should note that if you withdraw your acceptance of the offer, you will not receive any cash payment or amended options in replacement for the withdrawn options and those options will not be amended. If you withdraw your acceptance, you will keep all of your eligible options. These options will continue to be governed by the Aon Stock Option Plan, as amended, or the Aon Stock Incentive Plan, as amended, depending upon the Plan under which such options were granted, and by the existing option agreements between you and Aon. To the extent any portion of your eligible options vest or vested after December 31, 2004, you may be subject to the adverse personal tax consequences of Section 409A of the Internal Revenue Code described in the Offer to Amend and we will not reimburse you for any such taxes or any related penalties, charges or fees.

        You may change this withdrawal, and again elect to accept the offer with respect to all of your eligible options by submitting a new election form prior to the expiration date which will be 11:59 p.m., Chicago Time, on September 17, 2007, unless extended.

        Please check the box if you wish to withdraw your election:

o

I wish to withdraw my election to accept the offer with respect to all of my eligible options and instead REJECT the offer. I do not wish to accept the offer with respect to any of my eligible options.

        Please sign this withdrawal form and print your name exactly as it appears on the election form.

Employee Signature
Employee Name (Please print)	Company E-mail Address	Date and Time

To validly withdraw your election with respect to all of your eligible options, you must properly complete and sign the accompanying withdrawal form; and scan the properly completed withdraw and e-mail as a .pdf file to exec_compensation@aon.com or send the properly completed withdrawal form to
Aon by facsimile to 610-834-3390.

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AON CORPORATION
OFFER TO AMEND THE EXERCISE PRICE OF CERTAIN OPTIONS
INSTRUCTIONS TO THE WITHDRAWAL FORM
FORMING PART OF THE TERMS AND CONDITIONS OF THE OFFER

1.     Delivery of Withdrawal Form.

To validly withdraw your election with respect to all of your eligible options, you must properly complete and sign the accompanying withdrawal form; and scan the properly completed withdraw and e-mail as a.pdf file to exec_compensation@aon.com or send the properly completed withdrawal form to Aon by facsimile to 610-834-3390.

        Only withdrawal forms that are properly completed and submitted and actually received by the expiration date will be accepted. The expiration date will be 11:59 p.m., Chicago Time, on September 17, 2007, unless we extend the offer. Withdrawal forms that are received after the expiration date will not be accepted. Aon intends to confirm the receipt of your withdrawal form by e-mail within two business days. If you have not received an e-mail confirmation, you should confirm that we have received your withdrawal form. Only withdrawal forms that are properly completed and actually received by the expiration date will be accepted. Withdrawal forms submitted by any means other than those described above are not permitted without the prior consent of Jeremy Farmer, Senior Vice President and Head of Human Resources.

        Although by submitting a withdrawal form you have withdrawn all of the options with respect to which you previously chose to accept the offer from the offer, you may change your mind and re-elect to accept the offer with respect to all of the withdrawn options until the expiration of the offer. You should note that you may not rescind any withdrawal and any eligible options withdrawn will be deemed not to have had proper elections made for purposes of the offer, unless you properly re-elect to exchange those options before the expiration date. Re-elections with respect to options may be made at any time before the expiration date. If Aon extends the offer beyond that time, you may re-elect to participate with respect to your options at any time until the extended expiration of the offer. To re-elect with respect to all of the withdrawn options, you must deliver a later dated properly completed and electronically signed election form through Lotus Notes, while you still have the right to participate in the offer. You will not be deemed to have made a proper election with respect to your options for purposes of the offer unless you deliver, prior to the expiration date, a new election form following the procedures described in the instructions to the election form. This new election form must be signed and dated after your original election form and any withdrawal form you have submitted. Upon the receipt of such a new, properly completed, signed and dated election form, any previously submitted election form or withdrawal form will be disregarded and will be considered replaced in full by the new election form. You will be bound by the last properly submitted election or withdrawal form received by us prior to the expiration date.

        Although it is Aon's intent to send you an e-mail confirmation of receipt of this withdrawal form, by signing this withdrawal form, you waive any right to receive any notice of the withdrawal of your election with respect to your options.

2.     Signatures on This Withdrawal Form.

        If this withdrawal form is signed by the holder of the eligible options, the signature must correspond with the name as written on the face of the option agreement or agreements to which the options are subject without alteration or any change whatsoever. If your name has been legally changed since your option agreement was signed, please submit proof of the legal name change.

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        If this withdrawal form is signed by a trustee, executor, administrator, guardian, attorney-in-fact, officer of a corporation or other person acting in a fiduciary or representative capacity, that person should so indicate when signing, and proper evidence satisfactory to Aon of the authority of that person to act on your behalf must be submitted with this withdrawal form.

3.     Other Information on this Withdrawal Form.

        In addition to signing this withdrawal form, you must print your name and indicate the date and time at which you signed. You must also include your current Company e-mail address.

4.     Requests for Assistance or Additional Copies.

        If you need additional copies of the offer documents or the election or withdrawal forms or have questions regarding this offer, you should contact the Aon Tender Offer Call Center, at 1-888-295-1814. The hours of operation of the Aon Tender Offer Call Center are: 8:00 a.m. to 5:00 p.m. Chicago Time.

5.     Irregularities.

        We will determine, in our discretion, all questions as to the validity, form, eligibility, including time of receipt, and acceptance of any withdrawal forms. Our determination of these matters will be final and binding on all parties. We reserve the right to reject any withdrawal forms that we determine are not in appropriate form or that we determine are unlawful to accept. We also reserve the right to waive any of the conditions of the offer or any defect or irregularity in any withdrawal form or for any particular option holder, provided that if we grant any such waiver, it will be granted with respect to all option holders and tendered options. No withdrawal of options will be deemed to have been properly made until all defects or irregularities have been cured by the withdrawing option holder or waived by us. Neither we nor any other person is obligated to give notice of any defects or irregularities in withdrawal forms, nor will anyone incur any liability for failure to give any notice. This is a one-time offer. We will strictly enforce the offering period, subject only to an extension that we may grant in our sole discretion.

Important: The withdrawal form together with all other required documents must be received by Aon on or before the expiration date.

6.     Additional Documents to Read.

        You should be sure to read the Offer to Amend, all documents referenced therein (including the election form describing your eligible options and any potential cash payments), and the related memorandum from Jeremy Farmer, Senior Vice President and Head of Human Resources, dated August 15, 2007, before making any decisions regarding participation in, or withdrawal from, the offer.

7.     Important Tax Information.

        You should refer to Section 14 of the Offer to Amend, which contains important U.S. federal income tax information, including information relating to the tax consequences of not participating in this offer. We also recommend that you consult with your personal tax or financial advisor regarding Section 409A and state income tax considerations before deciding whether or not to participate in this offer.

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Exhibit (a)(1)(e)

EX-99.(A)(1)(F)

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Exhibit (a)(1)(f)

FORM OF
PROMISE TO MAKE CASH PAYMENT

TO:

        In exchange for your agreement to amend your eligible options as indicated by your election form, Aon hereby promises to pay you a cash payment for them as described on the schedule attached hereto. This payment will not be subject to any vesting conditions or otherwise be subject to forfeiture.

        The payment will be paid, less applicable tax withholding, on the first regular payroll date in January 2008.

        The payment due to you is shown on the attached schedule.

        This Promise to Make Cash Payment is subject to the terms and conditions of the offer as set forth in: (1) the Offer to Amend the Exercise Price of Certain Options (the "Offer to Amend") dated August 15, 2007; (2) the related memorandum from Jeremy Farmer, Senior Vice President and Head of Human Resources, dated August 15, 2007; (3) the election form; and (4) the withdrawal form (collectively, the "Offer Documents"), all of which are incorporated herein by reference. This Promise, the Offer Documents and the Amendment to Stock Option Agreements reflect the entire agreement between you and Aon with respect to this transaction. This Promise may be amended only by means of a writing signed by you and an authorized officer of Aon.

AON CORPORATION

By:

Date:

Title:

1

Schedule of Payment

Amended Option Number		Payment
	TOTAL:	$

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FORM OF PROMISE TO MAKE CASH PAYMENT
Schedule of Payment

EX-99.(A)(1)(G)

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Exhibit (a)(1)(g)

FORMS OF CONFIRMATIONS

Confirmation E-mail to Employees who Elect to Participate in the Offer to Amend the Exercise Price of Certain Options

        Aon Corporation has received your election form dated                        , 2007, by which you elected to have all of your eligible options amended, subject to the terms and conditions of the offer.

To validly withdraw your election with respect to all of your eligible options, you must properly complete and sign the accompanying withdrawal form; and scan the properly completed withdraw and e-mail as a .pdf file to exec_compensation@aon.com or send the properly completed withdrawal form to Aon by facsimile to 610-834-3390.

        If you have questions concerning the submission of your form, you should direct them to:

Aon Tender Offer Call Center
1-888-295-1814

        The hours of operation of the Aon Tender Offer Call Center are: 8:00 a.m. to 5:00 p.m. Chicago Time.

        Please note that our receipt of your election form is not by itself an acceptance of the options. For purposes of the offer, Aon will be deemed to have accepted options with respect to which proper elections have been made and not properly withdrawn as of the date when Aon gives written notice to the option holders generally of its acceptance of such options, which notice may be made by e-mail or other method of communication. Aon's formal acceptance is expected to take place shortly after the end of the offer period.

Confirmation E-mail to Employees who Withdraw their Election to Participate in the Offer to Amend the Exercise Price of Certain Options

        Aon Corporation has received your withdrawal form dated                        , 2007, by which you rejected Aon's offer to amend the Eligible Portion of all of your Eligible Options.

        If you change your mind, you may once again elect to accept the offer with respect to all of your eligible options by way of the e-mail message containing your election form, which you can complete, electronically sign and submit electronically through Lotus Notes. If you wish to participate in this offer, you must submit a properly completed and electronically signed election form through Lotus Notes prior to 11:59 p.m., Chicago Time, on September 17, 2007. Only election forms that are properly completed and submitted to us in this manner by the expiration date will be accepted. Election forms that are submitted after the expiration date will not be accepted. Aon intends to confirm the receipt of your election form and/or any withdrawal form by an e-mail message to you within two business days. If you have not received an e-mail confirmation, you should confirm that we have received your election form and/or any withdrawal form. Election forms submitted by any means other than those described above are not permitted without the prior consent of Jeremy Farmer, Senior Vice President and Head of Human Resources.

        If you have questions concerning the submission of your form, you should direct them to:

Aon Tender Offer Call Center
1-888-295-1814

        The hours of operation of the Aon Tender Offer Call Center are: 8:00 a.m. to 5:00 p.m. Chicago Time.

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FORMS OF CONFIRMATIONS

EX-99.(A)(1)(H)

Exhibit (a)(1)(h)

FORM OF INITIAL REMINDER E-MAIL TO ELIGIBLE EMPLOYEES

        As outlined in the documents included with the e-mail dated August 15, 2007, Aon Corporation has determined that certain options granted by Aon may subject the holder thereof to adverse personal tax consequences under Section 409A of the U.S. Internal Revenue Code ("Section 409A"). Section 409A imposes adverse tax consequences on stock options that were granted with exercise prices at a discount from fair market value and which vest after December 31, 2004. These tax consequences include income tax at vesting (as opposed to upon exercise), an additional 20% tax and interest charges. Those adverse tax consequences include subjecting the entire gain on the exercise of the discounted option to taxation.

        Aon has offered you the opportunity to participate in a tender offer for your eligible options. If you participate in the tender offer, your options will be amended so that they no longer will be subject to the adverse personal tax consequences. In addition, you will be eligible to receive a cash payment equal to the difference between the new exercise price of the amended portion of your eligible portion and the original exercise price, multiplied by the number of applicable unexercised shares (less applicable tax withholding).

        Note that you must act by September 17, 2007 and accept the offer to avoid the adverse tax impact. All of the details of Aon's offer to you are included in the e-mail dated August 15, 2007. For more information on how to participate in the offer you may contact:

Aon Tender Offer Call Center
1-888-295-1814

        The hours of operation of the Aon Tender Offer Call Center are: 8:00 a.m. to 5:00 p.m. Chicago Time.

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FORM OF ADDITIONAL REMINDER E-MAIL TO ELIGIBLE EMPLOYEES

        As previously outlined in the documents included with the e-mail dated August 15, 2007, Aon Corporation has determined that certain options granted by Aon may be subject the holder thereof to adverse personal tax consequences under Section 409A of the Internal Revenue Code ("Section 409A"). Section 409A imposes adverse tax consequences on stock options that were granted with exercise prices at a discount from fair market value and which vest after December 31, 2004. These tax consequences include income tax at vesting (as opposed to upon exercise), an additional 20% tax and interest charges. Those adverse tax consequences include subjecting the entire gain on the exercise of the discounted option to taxation.

        Aon has offered you the opportunity to participate in a tender offer for your eligible options. If you participate in the tender offer, your options will be amended so that they no longer will be subject to the adverse personal tax consequences. In addition, you will be eligible to receive a cash payment equal to the difference between the new exercise price of the amended portion of your eligible options and the original exercise price, multiplied by the applicable number of unexercised shares (less applicable tax withholding).

        As a reminder, you must act by September 17, 2007 and accept the offer to avoid the adverse tax impact. All of the details of Aon's offer to you are included in the e-mail dated August 15, 2007. For more information on how to participate in the offer you may contact:

Aon Tender Offer Call Center
1-888-295-1814

        The hours of operation of the Aon Tender Offer Call Center are: 8:00 a.m. to 5:00 p.m. Chicago Time.

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EX-99.(A)(1)(I)

Exhibit (a)(1)(i)

FORM OF AMENDMENT TO STOCK OPTION AGREEMENTS

AON CORPORATION

AMENDMENT TO STOCK OPTION AGREEMENTS

        WHEREAS, Aon Corporation (the "Company") and                        (the "Optionee") are parties to certain stock option agreements ("Agreements") granting Optionee options (the "Options") to purchase shares of Aon's common stock subject to the terms of the Aon Stock Incentive Plan, as amended and restated, and/or the Aon Stock Option Plan, as amended.

        WHEREAS, the Optionee previously accepted the offer made by Aon to amend the Eligible Portion of his or her Eligible Options (as identified on Exhibit A) pursuant to the terms of the Offer to Amend the Exercise Price of Certain Options dated August 15, 2007 and the Optionee's executed election form.

        NOW THEREFORE IT IS HEREBY AGREED AS FOLLOWS:

1.

Modification of Exercise Price.    As of                         , 2007, the Exercise Price per Share set forth in the Agreements shall be amended with regard to the Eligible Portion of the Eligible Options to the new Exercise Price per Share set forth on Exhibit A.

2.

Option Agreements.    To the extent not expressly amended hereby, the Agreements remain in full force and effect. Notwithstanding the foregoing, to the extent the Agreements made reference to the Options' qualification as Incentive Stock Options, the Optionee acknowledges that the Options, irrespective of this Amendment, will be reported as Non-Qualified Stock Options for United States tax law purposes.

3.

Entire Agreement.    This Amendment, taken together with the Agreements (to the extent not expressly amended hereby) and any duly authorized written agreement entered into by and between Aon and the Optionee relating to the Options evidenced by the Agreements, represent the entire agreement of the parties, supersede any and all previous contracts, arrangements or understandings between the parties with respect to the Option evidenced by the Agreements, and may be amended at any time only by mutual written agreement of the parties hereto.

        IN WITNESS WHEREOF, this instrument is executed as of September     , 2007.

OPTIONEE	AON CORPORATION
Signature	By:

Print Name
Title:

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Exhibit A

Eligible Option (Grant Number)	Original Date of Grant	Original Option Exercise Price	Shares Subject to Eligible Portion	Corrected Exercise Price for Eligible Portion	Cash Payment (Aggregate Price Differential)
		$		$	$
		$		$	$
						TOTAL $

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EX-99.(A)(1)(J)

Exhibit (a)(1)(j)

Presentation on Aon’s Tender Offer Program

Meeting Objectives Clarify which stock options were granted at a discount and are subject to adverse personal tax consequences Explain Aon's solution, through the tender offer, for impacted stock options Explain the benefit of participating in Aon’s solution Explain what happens if you choose not to participate

Why are we here today? Aon determined that some of your unexercised stock options were granted at a “discount” and are subject to adverse personal tax consequences (your “Eligible Options”) Aon has a solution that will allow you to avoid adverse personal tax consequences for the Eligible Options You have received an individualized statement identifying your Eligible Options

Why are we here today? To participate in the solution, you must make the election You are not required to participate If you do not participate, you will have adverse income tax consequences for which you will be solely responsible

How did I receive stock options subject to adverse personal tax consequences? Aon’s procedures relating to stock option grants caused incorrect measurement dates to be used for accounting and tax purposes Resulted in grants of discounted stock options Discounted stock options defined: The stock options are deemed discounted for accounting and tax purposes because the fair market value of Aon common stock on the “best evidence measurement date” was determined to be higher than the fair market value of Aon common stock – and exercise price - on the original grant date.

Which stock options are affected? The adverse tax consequences arise under Section 409A of the Internal Revenue Code. Stock options subject to 409A meet the following criteria: Stock options that are granted at an exercise price below the stock's fair market value ("FMV") on grant date AND Stock options that were unvested as of 12/31/04 and are currently outstanding or unexercised

Which stock options are affected? (continued) This means that all or a portion of your Option (i.e., only the portion or “tranche” of the stock option grant that was unvested as of 12/31/04) will be subject to adverse taxation. Aon refers to the affected portion of the stock option grant, to the extent such stock options are outstanding or unexercised today, as your “Eligible Options” You will receive an e-mail indicating which of your stock options are affected and eligible for the tender offer

What is Aon’s solution?

Aon's Solution: Tender Offer Aon is committed to providing a solution to fix the discounted stock options and make you whole as a recipient of these stock options What is the solution? 1. Amend the exercise price of the Eligible Options to the FMV of Aon common stock on the “best evidence measurement date” AND 2. Provide a cash payment to reimburse employees for the difference between the original exercise price and the amended exercise price.

Aon's Solution: Tender Offer Only the stock options impacted by adverse tax consequences require this solution and are eligible for cash payment US-based employees may keep the discount applicable to stock options not covered by Code Section 409A without adverse tax consequences Your consent to the offer permits Aon to implement the solution for you

Aon's Solution: Tender Offer Solution Implementation STEP 1 Amend unexercised Eligible Options to increase the exercise price Amended exercise price will equal the FMV on the “best evidence measurement date” All other terms and conditions will remain the same (including the number of shares, vesting schedule, and expiration date)

Aon's Solution: Tender Offer Solution Implementation STEP 2 Cash Payment Cash payment equal to: (Amended exercise price – Original exercise price) X (Number of stock options in the Eligible Options) Cash payment, less applicable withholding tax, will be made on the first payroll date following January 1, 2008. Timing is in accordance with the guidelines under US Code Section 409A.

Aon's Solution: Tender Offer Only the eligible portion of an Eligible Option may be amended through the tender offer: Unvested as of 12/31/04 Outstanding (unexercised) as of the expiration of the tender offer

Aon's Solution: Tender Offer How does the cash payment work related to the new grant price? 4/22/2002 Original Date of Grant 2,000 Shares Subject to Eligible Portion $2.37 Difference in Original and Amended Exercise Price $4,740 $39.25 $36.88 5/1/2002 Cash Payment (Aggregate Price Differential) Amended Exercise Price for Eligible Portion Original Exercise Price “Best Evidence Measurement Date”

Aon's Solution: Tender Offer The examples below illustrate the difference between accepting and declining the offer. Assumptions Options subject to 409A treatment: 1,000 Original exercise price: $25.00 Amended exercise price: $27.00 Market price at exercise: $38.00 In both examples, it is assumed that all Eligible Options are exercised at the market price of $38.00 in 2007. Net Pre-Tax Example 1: Options Value Gain Re-priced value (net value = market value – amended price) 1,000 $11.00 $11,000 Cash reimbursement for lost value (net value = amended price – original price) 1,000 $ 2.00 $ 2,000 Total Return (before income tax) $13,000 Net Pre-Tax Example 2: Options Value Gain Unadjusted gain (net value = market price – original price) 1,000 $13.00 $13,000 Federal excise tax 20% ($ 2,600) Total Return* (before income tax) $10,400 * Under 409A provisions, interest and penalties will also apply. Additional state law taxes may also apply, for example in California.

Who is Eligible for the Tender Offer? Must be an employee as of the expiration date of the tender offer and subject to personal income tax in the United States. Current and former executive officers of Aon are not eligible to participate in the tender offer. You cannot elect to tender just some of your Eligible Options--you must tender all of Eligible Options

What if I do nothing?

What if I do nothing? You will not receive the benefits of Aon's solutions: No amended option price No cash payment You will be responsible for the tax implications: Potential US federal income taxation prior to exercise 20% additional federal tax Potential interest due to late payment of federal income tax

What if I do nothing? There will be no reimbursement for taxes and interest incurred If you do not amend and do not exercise your Eligible Options, you will be taxed each year until exercise or expiration of the discounted options This solution is a ONE-TIME OFFER to prevent future adverse tax consequences

How do I participate?

What do I do to participate? Complete the Election Form you will be receiving through e-mail Complete your election through e-mail Deliver form by 11:59 p.m. CST on September 17 Election notices received after 11:59 p.m., September 17 will not be accepted. E-mail confirmation will be issued to you within 2 business days after receipt Participation in this tender offer is completely voluntary

Tender Offer Documents & Personalized Election Form You will receive an e-mail that includes: Personalized Election Form describing your Eligible Options Hyperlinks to: Copy of the Offer to Amend the Exercise Price of Certain Options, including a Q&A section to answer questions Withdrawal Form

Tender Offer Timeline Tender Offer Begins: August 15, 2007 Tender Offer Expires: September 17, 2007, unless extended at Aon’s sole discretion All elections must be received by 11:59 p.m. CST on September 17, 2007. Late submissions will not be accepted.

What if I still have questions? Contact: Aon Tender Offer Call Center: 1-888-295-1814 (8:00 am CST – 5:00 pm CST) Jacob Peters Elizabeth Woodruff

Important Information THIS PRESENTATION IS FOR INFORMATIONAL PURPOSES ONLY AND IS NOT AN OFFER TO PURCHASE OR THE SOLICITATION OF AN OFFER TO SELL ANY SECURITIES. THE SOLICITATION AND THE OFFER TO AMEND ARE MADE ONLY PURSUANT TO AN OFFER TO AMEND AND RELATED MATERIALS THAT AON FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ("SEC") ON AUGUST 15, 2007, AS PART OF AN OFFERING MEMORANDUM. OPTION HOLDERS WHO ARE ELIGIBLE TO PARTICIPATE IN THE TENDER OFFER SHOULD READ THE OFFER TO AMEND AND THE RELATED MATERIALS CAREFULLY BECAUSE THEY CONTAIN IMPORTANT INFORMATION ABOUT THE TENDER OFFER. OPTION HOLDERS MAY OBTAIN A COPY OF THE OFFER TO AMEND AND THE RELATED MATERIALS FREE OF CHARGE FROM THE SEC'S WEBSITE AT WWW.SEC.GOV OR FROM AON.

Circular 230 Disclaimer Notice Any tax advice included in this presentation was not intended or written to be used, and it cannot be used by the taxpayer, for the purpose of avoiding any penalties that may be imposed by any governmental taxing authority or agency. This tax advice was written to support the promotion of the matter addressed by the presentation. The taxpayer should seek advice based on the taxpayer's particular circumstances from an independent tax advisor.

EX-99.(D)(3)

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Exhibit (d)(3)

SECOND AMENDMENT
TO THE AON STOCK OPTION PLAN
AS AMENDED AND RESTATED THROUGH 1997

THIS SECOND AMENDMENT ("Second Amendment") TO THE AON STOCK OPTION PLAN AS AMENDED AND RESTATED THROUGH 1997 (the "Plan") is hereby made effective as of the 21^st day of January, 2000.

RECITALS

        WHEREAS, the Company deems it in the best interests of the Company to amend certain provisions of the Plan to clarify existing policies, practices and procedures; and

        WHEREAS, the Company desires to change the vesting schedule to reflect current competitive practices.

        NOW THEREFORE, the Plan is hereby amended as follows:

1.

Section 4 of the Plan is hereby amended by deleting the third and fourth paragraphs, in their entirety, and substituting the following:

A Grant shall vest after a Participant's period of continuous employment by the Corporation from the date of a Grant ("Grant Date") in accordance with the schedule set forth below:

Participant's Full Years of Continuous Employment from Grant Date	Percent Vested
2	33
3	34
4	33

A Participant may elect to exercise an option by giving written notice to the Corporation on such forms and in such manner as the Committee may prescribe. Payment for all shares to be purchased pursuant to an exercise of an option shall be made in a form or manner authorized by the Committee, including, but not limited to, cash or, if the Committee so permits, (i) by delivery of certification of ownership to the Corporation of the number of shares of stock which have been owned by the holder for at least six (6) months prior to the date of exercise having an aggregate Fair Market Value of not less than the product of the purchase price of the option multiplied by the number of shares the Participant intends to purchase upon exercise of the option on the date of delivery; or (ii) in a cashless exercise through a broker. Delivery of such certificates is conditioned on the Participant's prior compliance with this Section and with the terms of Section 7. Upon receipt of such stock certificate, the Participant is free to hold, or, subject to Section 13 dispose of it at will. The Participant does not have the right to vote any shares subject to a Grant or receive dividends on such shares prior to the time they are exercised. The Committee at its discretion may alter the terms of the vesting of Grants; provided, however, no Grant may be exercised after the tenth (10^th) anniversary of the date of the making of such Grant.

2.

Section 7 is hereby amended by adding the following sentence to the end of paragraph two:

However, in no event will the amount of shares withheld exceed the amount necessary to satisfy the required minimum statutory withholding.

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SECOND AMENDMENT TO THE AON STOCK OPTION PLAN AS AMENDED AND RESTATED THROUGH 1997
RECITALS

EX-99.(D)(7)

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Exhibit (d)(7)

IMPORTANT NOTICE—PLEASE READ BEFORE PROCEEDING

This grant must be accepted within ninety (90) days of the grant date in order to be eligible to receive any benefits from this grant. Refer to Section 11 Other Provisions, "Need to Accept Grant" for more information. Additionally, if this is your first grant of stock options from Aon, please make sure to submit a "Stock Option Beneficiary" form now. The form can be found at www.etrade.com/stockplans in the "Company Info" tab. The same form is also used to change your beneficiary.

AON CORPORATION
2001 AON STOCK INCENTIVE PLAN
STOCK OPTION AGREEMENT

        This Stock Option Award Agreement (the "Agreement") is entered into between Aon Corporation, a Delaware corporation (the "Company") and the employee (the "Employee") as listed on the "Notice of Grant of Stock Options" (the "Notice").

        The Company desires, by affording the Employee an opportunity to purchase shares of Aon's common stock (the "Common Stock") as hereinafter provided, to encourage the Employee to remain in the employ of the Company or its subsidiaries, to provide the Employee with an incentive to contribute to the financial progress of the Company, and to encourage ownership of the Company's stock by the Employee.

        NOW, THEREFORE, in consideration of the mutual promises hereinafter set forth, the parties hereto agree as follows:

1.

Grant of Option and Option Price.    The Company grants to the Employee under the 2001 Aon Stock Incentive Plan (the "Plan") the right and option ("Option") to purchase all or part of the number of shares of the Common Stock of the Company and at the Option price per share specified in the "Notice of Grant of Stock Options" (the "Notice").

2.

Notice of Grant of Stock Options.    The Notice shall specify the date of grant (the "Grant Date"), the Option price (the "Exercise Price"), the number of shares granted to the Employee, the expiration date of the Option, the vesting schedule of the Option, and the type of Option, specifying whether the grant is for non-qualified stock options ("NQ") or incentive stock options ("ISO"). The ISO only applies to Employees in the United States and is intended to qualify as an incentive stock option within the meaning of Section 422 of the United States Internal Revenue Code of 1986, as amended. The Notice is incorporated herein by reference and the terms of this Agreement are incorporated by reference in the Notice.

3.

Timing of Exercise.    Any portion of the Option may be exercised at any time after such options have become vested as long as the Employee remains employed, and for periods thereafter as indicated in Section 6 of this Agreement, but not to exceed the expiration date of the Option.

4.

Payment of Exercise Price.    The Employee shall at the time of exercise of an Option (except in the case of a cashless exercise) tender the full Exercise Price. At the discretion of the Committee, and subject to such rules and regulations as it may adopt, the Exercise Price may be paid: (i) in full in cash through means provided by the Company; (ii) by delivering irrevocable instructions to a broker to promptly deliver to the Company the amount of sale or loan proceeds to pay the Exercise Price; (iii) by attesting to the ownership of sufficient shares of Common Stock which have been held by the Employee for at least six months to pay the Exercise Price; or (iv) through a cashless exercise with a broker approved for this purpose by the Company.

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5.

Tax Deposit.    Upon exercising all or any part of an Option, the Employee shall deposit with the Company, through means provided for by the Company, an amount of cash equal to the amount determined by the Company to be withheld upon the exercise of the Option for any withholding taxes, social security / social insurance contributions, or the like under any government statute. The Committee may, at its sole discretion, and subject to such rules as it may adopt, permit the Employee to satisfy this withholding obligation, in whole or in part, by electing to sell shares of Common Stock having a fair market value on the date that the amount of tax to be withheld is determined equal to the applicable required minimum withholding. The Company shall not issue and deliver any of its Common Stock upon the exercise of any Option until and unless the Employee has made the deposit required herein or proper provision for withholding has been made.

6.     Effect of Termination of Employment.

a)

Voluntary termination prior to age 55.    The vested portion of the Option may be exercised no later than the 90-day period following termination of employment and the unvested portion will be forfeited.

b)

Voluntary termination on or after age 55.    The Option will be immediately vested pro rata, and may be exercised no later than three years from the date of termination (not to exceed the expiration date of the Option). The remaining unvested portion of the Option shall be forfeited. Pro rata vesting is based on the period of employment since the Grant Date.

c)

Termination due to disability or death.    The Option will be fully vested immediately and may be exercised no later one year from the date of termination (not to exceed the expiration date of the Option).

d)

Involuntary termination (other than for cause).    The Option will be immediately vested pro rata. The vested portion of the Option may be exercised no later than the 90-day period following termination of employment and the unvested portion will be forfeited. Pro rata vesting is based on the period of employment since the Grant Date.

e)

Termination for cause.    The Option will not be exercisable. Termination for cause shall mean performing an act of dishonesty, fraud, theft, embezzlement, or misappropriation involving Employee's employment with the Company, or breach of the duty of loyalty to the Company; performing an act of race, sex, national origin, religion, disability, or age-based discrimination which after investigation, counsel to the Company reasonably concludes will result in liability being imposed on the Company and / or Employee; material violation of Company policies and procedures including, but not limited to, the Aon Business Conduct Guidelines and the Aon Code of Ethics; material non-compliance with any terms of an Employment Agreement; or, performing an act resulting in a criminal felony charge brought against the Employee or a criminal conviction of Employee (other than conviction of a minor traffic violation).

7.

Receipt by the Employee of the Prospectus.    The Employee acknowledges receipt of the Plan prospectus that contains the entire Plan, and is incorporated herein by reference. The Employee represents and warrants that Employee has read the Plan and agrees that all Options awarded under it shall be subject to all of the terms and conditions of the Plan.

8.

Expiration Date of Option.    An Option awarded under the Plan shall expire as indicated on the "Notice of Grant of Stock Options", subject to the terms and conditions set forth in the Plan and this Agreement.

9.

Issuance of Shares.    The Employee shall have no interest in the shares covered by the Option grant contained in this Agreement unless and until shares are issued following the exercise of an Option.

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10.   Additional Covenants

a)

Non-Solicitation Covenant

(i)

Business Considerations.    The Company is in the business of providing insurance brokerage, reinsurance brokerage, conventional and alternative risk management products and services, benefits consulting, compensation consulting, human resources consulting, managing underwriting and related insurance services including accounting, claims management and handling, contract wording, information systems, actuarial services and the solicitation and servicing of individual and commercial clients. An essential element of its business is the development and maintenance of personal contacts and relationships with clients. Because of these contacts and relationships, it is common for the Company's clients to develop identification with the employee who services its insurance needs, rather than with the Company itself. The personal identification of clients of the Company with a Company employee creates the potential for the Employee's appropriation of the benefits of the relationships developed with clients on behalf of and at the expense of the Company. Since the Company would suffer irreparable harm if Employee left its employ and solicited the insurance or other related business of clients of the Company, it is reasonable to protect the Company against solicitation activities by Employee for a limited period of time after Employee leaves the Company so that the Company may renew or restore its business relationship with its client.

(ii)

Covenant Not to Solicit.    Employee hereby covenants and agrees that, except with the prior written consent of the Company, Employee will not for a period of two years after the end of employment compete directly or indirectly in any way with the business of the Company. For the purposes of this Agreement, "compete directly or indirectly in any way with the business of the Company" means to enter into or attempt to enter into (on Employee's own behalf or on behalf of any other person or entity) any business relationship of the same type or kind as the business relationship which exists between the Company and its clients or customers to provide services related to the business of the Company for any individual, partnership, corporation, association or other entity who or which was a client or customer for whom the Employee was the producer or on whose account Employee worked or became familiar during 24 months prior to the end of employment.

(iii)

Covenant Not to Hire.    The Employee hereby also agrees not to induce or attempt to induce, or to cause any person or other entity to induce or attempt to induce, any person who is an employee of the Company to leave the employ of the Company during the term of the covenant set forth in (ii) above.

(iv)

Acknowledgments.    The Company and the Employee acknowledge and agree that the covenants contained in (ii) and (iii) are reasonably necessary for the protection of the Company and are reasonably limited with respect to the activities they prohibit, their duration, their geographical scope and their effect on the Employee and the public. The parties acknowledge that the purpose and effect of the covenants simply are to protect the Company for a limited period of time from unfair competition by the Employee.

Nothing in this Agreement shall prohibit the Employee from obtaining a livelihood. The intent of the parties is that the restrictive covenant of non-solicitation by the Employee is limited to those clients and customers of the Company, as reflected by the books of the Company, during the 24 months prior to the end of Employee's employment with the Company.

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b)

Company's Right to Injunctive Relief; Attorneys' Fees.    The Employee acknowledges that the Employee's services to the Company are of a unique character which gives them a special value to the Company, the loss of which cannot reasonably or adequately be compensated in damages in an action at law, and that a breach of this Agreement will result in irreparable and continuing harm to the Company, and that therefore, in addition to any other remedy which the Company may have at law or in equity, the Company shall be entitled to injunctive relief for a breach of this Agreement by Employee. In the event that the Company brings an action to enforce the terms and conditions of this Agreement, Employee shall pay the costs and expenses incurred by the Company in bringing such action, including legal fees.

c)

Trade Secrets and Confidential Information.    Employee acknowledges that the Company's business depends to a significant degree upon the possession of information which is not generally known to others, and that the profitability of such business requires that this information remain proprietary to the Company. The Employee shall not, except as required in the course of employment by the Company, disclose or use during or subsequent to the course of employment, any trade secrets or confidential or proprietary information relating to the business of the Company of which the Employee becomes aware by reason of being employed or to which Employee gains access during his employment by the Company and which has not been publicly disclosed (other than by Employee in breach of this provision).

Such information includes client and customer lists, data, records, computer programs, manuals, processes, methods and intangible rights which are either developed by the Employee during the course of employment or to which the Employee has access. All records and equipment and other materials relating in any way to any confidential information relating to clients or business of the Company shall be and remain the sole property of the Company during and after the end of employment.

d)

In the event this program is determined to be a "deferred compensation plan" subject to Section 409A of the Internal Revenue Code of 1986, as amended, the Company shall as necessary adopt such conforming amendments as are necessary to comply with Section 409A.

11.   Other Provisions

a)

Plan Terms Take Precedence over Agreement Terms.    Options are granted pursuant to the Plan, the terms and condition of which are incorporated into this Agreement by reference. If there are any inconsistencies between the terms of this Agreement and the Plan, the terms of the Plan will govern.

b)

Prior Agreement(s) Will Not Control.    Employee's acceptance of this Agreement will supersede provisions of any prior agreement that could be construed as governing the terms of this grant.

c)

Nontransferability.    Except as described below, ISOs and NQs are not transferable by the Employee other than by will, the laws of descent and distribution, or pursuant to a beneficiary designation. Except as described below, any ISO or NQ is exercisable, during the Employee's lifetime, only by the Employee or Employee's legal representative. The ISOs and NQs may not be pledged, mortgaged, hypothecated or otherwise encumbered, or subject to the claims of creditors, or assigned pursuant to any domestic relations order. The Employee may transfer all or a portion of a NQ for no consideration to or for the benefit of the Employee's immediate family (spouse, parents, children, stepchildren, adopted children, siblings and grandchildren). Such transfer may be directly to immediate family members, to a partnership, limited liability company or trust for the benefit of one or more immediate family members, or any other transfer deemed to be consistent with such a transfer.

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d)

Right of Employment.    Grants of Stock Options under the Plan and of this Agreement do not confer upon Employee any right to continue in the employ of the Employer.

e)

Beneficiary.    An Employee's "beneficiary" means the person(s) or entity as designated by Employee in the most recent written beneficiary designation form filed with the Company to receive the benefits specified under the Plan upon the death of the Employee, or, if there is no designated beneficiary or surviving designated beneficiary, then the estate of the Employee.

f)

Data Privacy.    Employee understands and authorizes Employer to share Employee's personal data with the Company, the U.S. parent company. Employee also understands and authorizes that this data, as listed below, will be shared with third party vendors hired by the Company to assist in administering the Plan. Employee consents to the Employee's Employer sharing of personal data (i.e. identification data, including name, address, telephone; financial data, including account numbers, wages; personal data, including age, gender, date of birth; education related data, including academic curriculum, professional experience; profession related data, including title and description of functions with the Company). Employee also authorizes Employer and the Company to receive, possess, use, retain, and transfer the data, in electronic form or other, and to further transfer data to third party vendors for purposes of assisting in the administration and managing Employee's participation in the Plan.

g)

Need to Accept Grant.    Employee acknowledges that this grant must be accepted within ninety (90) days of the Grant Date in order to be eligible to receive any benefits from this grant. If this grant is not accepted within the ninety (90) days, the grant will be cancelled and all benefits under this grant will be forfeited. To accept this grant, the Employee must access the www.etrade.com/stockplans website and follow the instructions for acceptance. If this grant was distributed to the Employee via mail, Employee must sign the agreement and return it to Aon's Executive Compensation Department within ninety (90) days.

h)

Computation of Severance / Retirement Benefits.    Benefits and rights acquired under the Plan do not constitute "base salary" or other regular employment earnings. Accordingly, Employee understands and accepts that benefits provided under the Plan will not be considered in calculating any of the Company's and its subsidiaries' obligations to Employee for bonus, retirement, severance, termination, health and welfare, or any other such payments, unless otherwise specified in the applicable plan.

i)

Plan Changes / Acquired Rights.    Employee understands and agrees that the Company may terminate, change or otherwise alter the terms and conditions of the Plan at any time, and that any such termination, change or alteration will not amount to a breach or breaches, fundamental or otherwise, of Employee's terms and conditions of employment. The scope of any change in terms is unforeseen; however, potential changes to the Plan may include, but are not limited to, 1) alteration of the discount at which employees are allowed to acquire Company shares, 2) modification of the vesting and/or offering periods, 3) adjustment of the award amounts, and 4) cancellation of the Plan. Employee hereby elects to participate in the Plan with full knowledge that benefits under the Plan can be terminated or otherwise modified by the Company at its sole discretion at any time.

j)

Waiver.    Waiver of any term or condition of this Agreement by any party shall not be construed as a waiver of a subsequent breach or failure of the same term or condition, or a waiver of any other term or condition of this Agreement. Any waiver must be in writing.

k)

Severability.    To the extent that the terms set forth in this Agreement or any word, phrase, clause or sentence is found to be illegal or unenforceable for any reason, such word, phrase, clause or sentence shall be modified or deleted in such manner so as to afford the Company the fullest protection commensurate with making this Agreement, as modified, legal and

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enforceable under applicable laws, and the balance of this Agreement shall not be affected thereby, the balance being construed as severable and independent.

l)

Governing Law.    The validity, interpretation, instruction, performance, enforcement and remedies of or relating to this Agreement, and the rights and obligations of the parties hereunder, shall be governed by and construed in accordance with the substantive laws of the State of Illinois, without regard to the conflict of law principles, rules or statutes of any jurisdiction. For Employees outside of the United States, this Agreement shall be governed by the applicable regulations or international treaty.

m)

Notice.    All notices given hereunder shall be in writing and, if intended for the Company, shall be addressed to it or delivered to it at its principal office to the attention of Executive Compensation Department. If intended for the Employee, notices shall be delivered personally or shall be addressed (if sent by mail) to the Employee's then current residence address as shown on the Company's records, or to such other address as the Employee directs in a notice to the Company. All notices shall be deemed to be given on the date received at the address of the addressee or, if delivered personally, on the date delivered.

IN WITNESS WHEREOF, the parties have signed this Agreement as of the date hereof.

AON CORPORATION
Gregory C. Case
President and Chief Executive Officer
(Accept grant online via www.etrade.com/stockplans account)Option Recipient (Employee)	Date

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Exhibit (d)(7)

AON CORPORATION 2001 AON STOCK INCENTIVE PLAN STOCK OPTION AGREEMENT