UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, D.C. 20549 
FORM 11-K 

x	 	Annual Report Pursuant to Section 15(d) of the Securities	
 	 	Exchange Act of 1934	
 	 	 	
 	 	For the fiscal year ended December 31, 2005	
 	 	OR	
o	 	Transition Report Pursuant to Section 15(d) of the	
 	 	Securities Exchange Act of 1934	
 	 	Commission File number 1-7933	
 
A.              Full title of the plan and the address of the plan, if different from that of the issuer named below:
Aon Savings Plan
B.                Name of issuer of the securities held pursuant to the plan and the address of its principal executive office:
Aon Corporation
200 E. Randolph Drive
Chicago, Illinois 60601

 
S I G N A T U R E S
Pursuant to the requirements of the Securities Exchange Act of 1934, the Committee acting as Plan Administrator, has duly caused this annual report to be signed on its behalf by the undersigned, hereunto duly authorized.
AON SAVINGS PLAN
BY THE COMMITTEE

/s/ Thomas Stachura	 	/s/ John A. Reschke	
Thomas Stachura	 	John A. Reschke	
 
Date:
June 28, 2006

2

FINANCIAL STATEMENTS AND SUPPLEMENTAL SCHEDULE 
AON SAVINGS PLAN 
Years Ended December 31, 2005 and 2004 
With Report of Independent Registered 
Public Accounting Firm 
Employer Identification Number  36-3051915 
Plan # 020 

 
AON SAVINGS PLAN
FINANCIAL STATEMENTS AND SUPPLEMENTAL SCHEDULE
Years ended December 31, 2005 and 2004
CONTENTS

Report of Independent Registered Public Accounting Firm	 	1	 	
Financial Statements	 	 	 	
Statements of Assets Available for Benefits	 	 	 	
at December 31, 2005 and 2004	 	2	 	
Statements of Changes in Assets Available for Benefits	 	 	 	
for the years ended December 31, 2005 and 2004	 	3	 	
Notes to Financial Statements	 	4	 	
Supplemental Schedule:	 	 	 	
Schedule H, Line 4i — Schedule of Assets (Held At  	 	 	 	
End of Year)	 	10	 	

 
Report of Independent Registered Public Accounting Firm 
The Participants and Administrative Committee
Aon Savings Plan
We have audited the accompanying statements of assets available for benefits of the Aon Savings Plan as of December 31, 2005 and 2004, and the related statements of changes in assets available for benefits for the years then ended. These financial statements are the responsibility of the Plan’s management. Our responsibility is to express an opinion on these financial statements based on our audits.
We conducted our audits in accordance with the standards of the Public Company Accounting Oversight Board (United States). Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement. We were not engaged to perform an audit of the Plan’s internal control over financial reporting. Our audits included consideration of internal control over financial reporting as a basis for designing audit procedures that are appropriate in the circumstances, but not for the purpose of expressing an opinion on the effectiveness of the Plan’s internal control over financial reporting. Accordingly, we express no such opinion. An audit also includes examining, on a test basis, evidence supporting the amounts and disclosures in the financial statements, assessing the accounting principles used and significant estimates made by management, and evaluating the overall financial statement presentation. We believe that our audits provide a reasonable basis for our opinion.
In our opinion, the financial statements referred to above present fairly, in all material respects, the assets available for benefits of the Plan at December 31, 2005 and 2004, and the changes in its assets available for benefits for the years then ended, in conformity with U.S. generally accepted accounting principles.
Our audits were performed for the purpose of forming an opinion on the basic financial statements taken as a whole. The accompanying supplemental schedule of assets (held at end of year) as of December 31, 2005, is presented for purposes of additional analysis and is not a required part of the financial statements, but is supplementary information required by the Department of Labor’s Rules and Regulations for Reporting and Disclosure under the Employee Retirement Income Security Act of 1974. This supplemental schedule is the responsibility of the Plan’s management. The supplemental schedule has been subjected to the auditing procedures applied in our audits of the basic financial statements and, in our opinion, is fairly stated in all material respects in relation to the basic financial statements taken as a whole.

Chicago, Illinois	 	/s/ ERNST & YOUNG LLP	
June 6, 2006	 	 	
 
1

	 	Employer Plan Identification # 36-3051915	
 	 	Plan # 020	
 
AON SAVINGS PLAN 
Statements of Assets Available for Benefits 

 (in thousands) 

 	 	December 31	 	
 	 	2005	 	2004	 	
Assets:	 	 	 	 	 	
Investments, at Fair Value:	 	 	 	 	 	
Aon Corporation Common Stock	 	$	324,206	 	$	258,761	 	
Brokerage Accounts-Other Common and Preferred Stocks and Mutual Funds	 	11,257	 	9,215	 	
Investments held in Mutual Funds	 	 	 	 	 	
State Street Global Advisors Short Term Investment Fund	 	250,649	 	254,613	 	
Vanguard REIT Index Fund	 	82,888	 	74,396	 	
Vanguard Admiral Intermediate Term Treasury Fund	 	71,187	 	73,341	 	
Vanguard Capital Opportunities Fund	 	71,764	 	60,849	 	
T. Rowe Price Growth Stock Fund	 	29,225	 	—	 	
Dodge & Cox Common Stock Fund	 	199,629	 	174,806	 	
PIMCO Total Return Fund	 	53,863	 	49,788	 	
Montag & Caldwell Growth Fund	 	—	 	23,285	 	
Managers Special Equity Fund	 	51,454	 	53,183	 	
Wells Fargo Small Cap Value Fund	 	15,256	 	—	 	
American Funds Euro-Pacific Growth Fund	 	102,110	 	77,495	 	
Investments held in Collective Trusts	 	 	 	 	 	
Ned Davis Research Asset Allocation Strategy Fund	 	129,125	 	138,855	 	
State Street Global Advisors S&P 500 Strategy Fund	 	243,785	 	255,803	 	
Short-Term Investment Funds	 	2,086	 	1,943	 	
Participant Loans	 	23,686	 	23,335	 	
 	 	1,662,170	 	1,529,668	 	
Contributions Receivable	 	 	 	 	 	
Participant Contribution Receivable	 	2,508	 	2,517	 	
Company Contribution Receivable	 	17,824	 	9,219	 	
 	 	20,332	 	11,736	 	
Investments Receivable:	 	 	 	 	 	
Dividends declared - American Funds Euro-Pacific Growth Fund	 	5,293	 	—	 	
Assets Available for Benefits	 	1,687,795	 	$	1,541,404	 	
								
 See notes to financial statements.
 
2

 

	 	Employer Plan Identification # 36-3051915	
 	 	Plan # 020	
 
AON SAVINGS PLAN 
Statements of Changes in Assets Available for Benefits 

 (in thousands) 

 	 	December 31	 	
 	 	2005	 	2004	 	
Net Investment Income	 	 	 	 	 	
Interest Income	 	$	10,712	 	$	3,574	 	
Net Appreciation in Fair Value of Investments	 	162,380	 	89,855	 	
Aon Corporation Dividends	 	6,201	 	7,145	 	
Other Dividends	 	31,610	 	21,339	 	
Total Net Investment Income	 	210,903	 	121,913	 	
Contributions	 	 	 	 	 	
Company	 	48,030	 	40,202	 	
Participants	 	90,127	 	100,885	 	
Rollovers	 	5,390	 	5,573	 	
Total Contributions	 	143,547	 	146,660	 	
Other Changes	 	 	 	 	 	
Benefit Payments	 	(206,007	)	(144,326	)	
Management and Administrative Fees	 	(2,052	)	(1,580	)	
Total Other Changes	 	(208,059	)	(145,906	)	
Net Increase in Assets Available for Benefits	 	146,391	 	122,667	 	
Assets Available for Benefits at Beginning of Year	 	1,541,404	 	1,418,737	 	
Assets Available for Benefits at End of Year	 	$	1,687,795	 	$	1,541,404	 	
 
See notes to financial statements.
3

	Employer Plan Identification # 36-3051915	
 	Plan # 020	
 
AON SAVINGS PLAN
NOTES TO FINANCIAL STATEMENTS 
 
Years Ended December 31, 2005 and 2004
 
1.                                       Description of Plan
                General
The Aon Savings Plan (the Plan) was authorized by the Board of Directors of Aon Corporation (the Company or Plan Sponsor). It is a defined contribution plan with a salary deferral feature and an employee stock ownership (ESOP) feature. The Plan is subject to the provisions of the Employee Retirement Income Security Act of 1974 (ERISA). Effective January 1, 2004, the Aon Retirement Account was established as a separate account under the Aon Savings Plan. The Aon Retirement Account is intended for employees hired after January 1, 2004 who are not eligible for participation in the Aon Pension Plan.
Effective January 1, 2003, the Aon Common Stock Fund and the ESOP Allocated Fund were merged into a single fund called the Aon Common Stock ESOP Fund. Participants have the option to reinvest dividends in additional shares of Aon common stock in the Plan or receive dividends in cash. Additionally, effective January 1, 2003, participants are allowed to immediately diversify any Company matching contributions allocated to the Aon Common Stock ESOP Fund (ESOP Fund).
The following description of the Plan provides only general information. Participants of the Plan should refer to the Summary Plan Description for a more complete description of the Plan.
Eligibility and Participation
Employees other than field sales agents or employees scheduled to work less than 20 hours per week are immediately eligible to participate. Field sales agents and employees scheduled to work less than 20 hours per week are eligible to participate after completing one year of service. Participants must complete one year of service to be eligible for Company matching contributions.
4

                Contributions
Participant — Participant contributions are made by means of regular payroll deductions. Non-highly compensated participants, as defined by the Internal Revenue Code (IRC), may elect to make contributions between 1% and 25% (16% prior to January 1, 2004) of their compensation, as defined by the Plan. Highly compensated participants, as defined by the IRC, may elect to make contributions between 1% and 10% (8% prior to January 1, 2004) of their compensation, as defined by the Plan. Participant contributions are limited to amounts allowed by the Internal Revenue Service (IRS). Accordingly, the maximum participant contribution was $14,000 in 2005 and $13,000 in 2004. In addition to regular participant contributions, catch-up contributions of up to $4,000 for 2005 and $3,000 for 2004 were allowed for any participants who were age 50 or older during the Plan year.
The Aon Retirement Account is funded entirely by Company contributions. No employee contributions are allowed. The Plan does not guarantee Company contributions however it is intended the Company will make an annual contribution to the account of eligible employees. The amount of the contribution may increase with length of service of the employee and other factors deemed relevant by the Board.
Company — The Company contributes an amount equal to 50% of the first 6% of a participant’s compensation that a participant contributes to the Plan (75% of a participant’s compensation up to 4% for employees of Aon Human Capital Services, LLC and 100% of the first 3% for employees of Human Resources Outsourcing Group). This contribution will be made concurrent with participant contributions. Employees of the Human Resources Outsourcing Group receive a further contribution equal to 75% of the next 3% of compensation contributed to the Plan. The Company may make a further discretionary contribution based on employee contributions of up to 6%. The amount of this contribution is determined by the Aon Board of Directors.
Investment Options
Both participant and Company contributions to the Plan will be invested in any of the various investment alternatives offered by the Plan in any whole percentages as directed by the participant.  Additionally, a Self-Managed Account is offered whereby participants can invest their self-directed contributions in various stock, mutual funds and other investments.
5

Participant Accounts
Each participant’s account is credited with the participant’s contribution and allocations of a) the Company’s contributions and b) Plan earnings (losses). The benefit to which a participant is entitled is the benefit that can be provided from the participant’s account.
Vesting
Participants are fully vested in their contributions plus actual earnings of the Plan. Participants become 100% vested in the employer contributions (excluding amounts in the Aon Retirement Account) after five years of plan service, according to a graded vesting schedule.
Participants become fully vested in the Aon Retirement Account after five years of service
Forfeitures of $2,300,000 for 2005, and $1,538,000 for 2004 were used to provide partial funding for Company contributions and to pay other expenses of the Plan.
Benefit Payments
Upon retirement or termination of service, a participant will receive a lump sum payment equal to his or her vested balance. The participant may elect to receive this payment directly or to be rolled into another Plan or IRA. Vested amounts of the ESOP may be received in cash or Aon Common stock.
Plan Termination
Although it has not expressed any intent to do so, the Company has the right under the Plan to discontinue its contributions at any time and to terminate the Plan subject to the provisions of ERISA. In the event of Plan termination, participants will become 100% vested in their accounts.
Participant Loans
Under the loan provision of the Plan, each participant is permitted one loan in a twelve month period and the outstanding balance of all loans made to a participant may not exceed the lesser of $50,000 or 50% of the vested portion of the participant’s account, excluding the ESOP and Aon Retirement Account portion of the account. The interest rate for each loan is equal to 1% plus the prime rate as quoted in The Wall Street Journal for the last day of the month preceding the loan request. Loans are made for a period of up to five years, except for residential loans that have a fixed repayment period of up to fifteen years.
6

                Basis of Accounting
The financial statements of the Plan are prepared on an accrual basis in accordance with U.S. generally accepted accounting principles.
Investment Valuation and Income Recognition
Investments in mutual funds and common stock are carried at fair value, which for marketable securities is based on quotations obtained from national securities exchanges. Investments in collective trusts are carried at redemption value as determined by the applicable trust on the last day of the Plan year. Participant loans are valued at their outstanding balances, which approximate fair value.
Interest income is recorded as earned. Dividend income is recorded on the ex-dividend date. Realized gains or losses on investments are the difference between the proceeds received and the cost of investments sold as determined on a first-in, first-out basis. The change in the difference between fair value and the cost of investments is reported as unrealized appreciation or depreciation of investments.
Administrative Expenses
Administrative expenses of the Plan, including expenses of the Trustees, are paid from the Plan assets, except to the extent that the Company, at its discretion, may decide to pay such expenses. The Company did not pay any Plan expenses in 2005 or 2004.
Use of Estimates
The preparation of the financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates.
3.                                       Investments
Effective October 1, 2004, State Street Bank and Trust Company has been appointed Trustee and custodian for all Plan assets. Prior to this date, Institutional Trust Company (ITC) was custodian of the Plan assets and Trustee for all Plan assets except the Aon Common Stock ESOP shares and the self-managed brokerage accounts. For the Aon Common Stock ESOP shares and self-managed brokerage accounts, State Street Bank and Trust Company was the Trustee. The Trustees are named fiduciaries under ERISA. As used herein, Trustee refers to  ITC and State Street Bank and Trust Company individually and collectively.
7

During 2005 and 2004 the Plan’s investments (including investments bought, sold and held during the year) appreciated (depreciated) in fair value as follows (in thousands):

 	 	December 31	 	
 	 	2005	 	2004	 	
 	 	Fair
Value	 	Net
Realized and
Unrealized
Appreciation
(Depreciation)
in Fair
Value of
Investments
During the
Year	 	Fair
Value	 	Net
Realized and
Unrealized
Appreciation
(Depreciation)
in Fair
Value of
Investments
During the
Year	 	
Investments, at fair value:	 	 	 	 	 	 	 	 	 	
Aon Corporation Common Stock	 	$	324,206	 	$	115,249	 	$	258,761	 	$	606	 	
Brokerage Accounts-Other Common and Preferred Stocks and Mutual Funds	 	11,257	 	391	 	9,215	 	(38	)	
Investments in Mutual Funds:	 	 	 	 	 	 	 	 	 	
State Street Global Advisors Short Term Investment Fund	 	250,649	 	-	 	254,613	 	-	 	
AIM Liquid Asset Fund	 	-	 	-	 	-	 	-	 	
Vanguard REIT Index Fund	 	82,888	 	4,544	 	74,396	 	13,335	 	
Vanguard Capital Opportunities Fund	 	71,764	 	5,248	 	60,849	 	6,851	 	
Wells Fargo Small Cap Value Fund	 	15,256	 	(880	)	-	 	803	 	
Dodge & Cox Common Stock Fund	 	199,629	 	10,598	 	174,806	 	19,224	 	
PIMCO Total Return Fund	 	53,863	 	(970	)	49,788	 	(199	)	
Montag & Caldwell Growth Fund	 	-	 	-	 	23,285	 	785	 	
Managers Special Equity Fund	 	51,454	 	(1,878	)	53,183	 	6,521	 	
T. Rowe Price Growth Stock Fund	 	29,225	 	2,460	 	-	 	(55	)	
American Euro-Pacific Growth Fund	 	102,110	 	13,087	 	77,495	 	9,692	 	
Vanguard Admiral Intermediate Term Treasury Fund	 	71,187	 	(2,594	)	73,341	 	(1,009	)	
Investments in Collective Trusts:	 	 	 	 	 	 	 	 	 	
State Street Asset Allocation Fund	 	-	 	-	 	-	 	(1,901	)	
Ned Davis Research Asset Allocation Fund	 	129,125	 	4,806	 	138,855	 	7,739	 	
State Street Global Advisors S&P 500 Strategy Fund	 	243,785	 	12,319	 	255,803	 	21,150	 	
INVESCO Asset Allocation Fund	 	-	 	-	 	-	 	1,653	 	
INVESCO 500 Index Fund	 	-	 	-	 	-	 	4,220	 	
INVESCO International Equity Fund	 	-	 	-	 	-	 	478	 	
Short-Term Investment Funds	 	2,086	 	-	 	1,943	 	-	 	
Total	 	$	1,638,484	 	$	162,380	 	$	1,506,333	 	$	89,855	 	
 
8

The fair value of individual investments that represent 5% or more of the Plan’s assets is as follows (in thousands):

	 	December 31	 	
 	 	2005	 	2004	 	
Aon Corporation, Common Stock	 	$	324,206	 	$	258,761	 	
Investments in Mutual Funds:	 	 	 	 	 	
State Street Global Advisors Short Term Investment Fund	 	250,649	 	254,613	 	
Dodge & Cox Common Stock Fund	 	199,629	 	174,806	 	
American Euro-Pacific Growth Fund	 	102,110	 	77,495	 	
Investments in Collective Trusts:	 	 	 	 	 	
Ned Davis Research Asset Allocation Strategy Fund	 	129,125	 	138,855	 	
State Street Global Advisors S&P 500 Strategy Fund	 	243,785	 	255,803	 	

4.                                       Income Tax Status
The Plan has received a determination letter from the IRS dated October 1, 2003, stating that the Plan is qualified under section 401(a) of the IRC and, therefore, the related trust is exempt from taxation. Subsequent to this issuance of the determination letter, the Plan was amended. Once qualified, the Plan is required to operate in conformity with the IRC to maintain its qualification. The Plan Administrative Committee believes the Plan is being operated in compliance with the applicable requirements of the IRC and, therefore, believes that the Plan, as amended, is qualified and the related trust is tax exempt.
5.                                       Risks and Uncertainties
The Plan invests in various investment securities. Investment securities are exposed to various risks such as interest rate, market and credit risks. Due to the level of risk associated with certain investment securities, it is at least reasonably possible that changes in the values of investment securities will occur in the near term and that such changes could materially affect participants’ account balances and the amounts reported in the statements of assets available for benefits.
.
 
9

 	 	Employer Identification # 36-3051915	
 	 	Plan # 020	
 
AON SAVINGS PLAN
Schedule H, Line 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) 
December 31, 2005

Identity of Issuer	 	 	 	Number of
Shares
Principal
Amount	 	Current Value
(thousands)	 	
 	 	 	 	 	 	
Aon Common Stock ESOP Fund	 	 	 	 	 	
 	 	 	 	 	 	
Common Stock	 	 	 	 	 	
 	 	 	 	 	 	
Aon Corporation	 	 	 	 	 	
Common Stock, 1.00 par*	 	9,016,429	 	$	324,141	 	
 	 	 	 	 	 	
Short Term Investment Fund	 	 	 	 	 	
 	 	 	 	 	 	
Mutual Fund	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the State Street Global	 	 	 	 	 	
Advisor Short Term Investment Fund*	 	250,649,273	 	$	250,649	 	
 	 	 	 	 	 	
Total Return Fund	 	 	 	 	 	
 	 	 	 	 	 	
Collective Trust	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the Ned Davis Research	 	 	 	 	 	
Asset Allocation Fund	 	9,877,656	 	$	129,125	 	
 	 	 	 	 	 	
Common Stock Index Fund	 	 	 	 	 	
 	 	 	 	 	 	
Collective Trust	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the State Street Global	 	 	 	 	 	
Advisors S&P 500 Strategy Fund*	 	1,051,643	 	$	243,785	 	
 	 	 	 	 	 	
Real Estate Securities Fund	 	 	 	 	 	
 	 	 	 	 	 	
Mutual Fund	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the Vanguard	 	 	 	 	 	
REIT Index Fund	 	6,351,492	 	$	82,888	 	
 
10

 

Vanguard Capital Opportunities Fund	 	 	 	 	 	
 	 	 	 	 	 	
Mutual Fund	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the Vanguard Capital	 	 	 	 	 	
Opportunities Fund	 	940,943	 	$	71,764	 	
 	 	 	 	 	 	
Dodge & Cox Common Stock Fund	 	 	 	 	 	
 	 	 	 	 	 	
Mutual Fund	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the Dodge & Cox	 	 	 	 	 	
Common Stock Fund	 	1,454,945	 	$	199,629	 	
 	 	 	 	 	 	
PIMCO Total Return Fund	 	 	 	 	 	
 	 	 	 	 	 	
Mutual Fund	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the PIMCO	 	 	 	 	 	
Total Return Fund	 	5,108,825	 	$	53,863	 	
 	 	 	 	 	 	
T. Rowe Price Growth Stock Fund	 	 	 	 	 	
 	 	 	 	 	 	
Mutual Fund	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the T. Rowe Price	 	 	 	 	 	
Growth Stock Fund	 	1,022,723	 	$	29,225	 	
 	 	 	 	 	 	
Managers Special Equity Fund	 	 	 	 	 	
 	 	 	 	 	 	
Mutual Fund	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the Managers	 	 	 	 	 	
Special Equity Fund	 	595,876	 	$	51,454	 	
 
11

 

Vanguard Admiral Intermediate Term Treasury Fund	 	 	 	 	 	
 	 	 	 	 	 	
Mutual Fund	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the Vanguard Admiral	 	 	 	 	 	
Intermediate Term Treasury Fund	 	6,485,841	 	$	71,187	 	
 	 	 	 	 	 	
American Euro-Pacific Growth Fund	 	 	 	 	 	
 	 	 	 	 	 	
Mutual Fund	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the American Euro-	 	 	 	 	 	
Pacific Growth Fund	 	2,595,548	 	$	102,110	 	
 	 	 	 	 	 	
Wells Fargo Small Cap Value Fund	 	 	 	 	 	
 	 	 	 	 	 	
Mutual Fund	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in the Wells Fargo Small	 	 	 	 	 	
Cap Value Fund	 	503,272	 	$	15,256	 	
 	 	 	 	 	 	
Other Common and Preferred Stocks and Mutual Funds (Self-Managed Funds)	 	 	 	 	 	
 	 	 	 	 	 	
Brokerage Accounts	 	 	 	 	 	
 	 	 	 	 	 	
Investments held in other common and preferred	 	 	 	 	 	
stocks and mutual funds and other investments	 	11,257,051	 	$	11,257	 	
 	 	 	 	 	 	
Investments held in Aon Corporation	 	 	 	 	 	
Common Stock*	 	1,799	 	$	65	 	
 	 	 	 	 	 	
Investment held in the State Street	 	 	 	 	 	
Short-Term Investment Fund	 	2,086,157	 	$	2,086	 	
 	 	 	 	 	 	
Participant Loans (5.00% - 10.5%)	 	—	 	$	23,686	 	
 	 	 	 	 	 	
 	 	 	 	$	1,662,170	 	
 
*Party in interest transaction not prohibited by ERISA.
 
12

 

EXHIBIT 23

Consent of Independent Registered Public Accounting Firm
We consent to the incorporation by reference in the Registration Statement (Form S-8 No. 33-27894) pertaining to the Aon Savings Plan of Aon Corporation of our report dated June 6, 2006, with respect to the financial statements and schedule of the Aon Savings Plan included in this annual report (Form 11-K) for the year ended December 31, 2005.
 
/s/ ERNST & YOUNG LLP
Chicago, Illinois
June 26, 2006