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SECURITIES AND EXCHANGE COMMISSION
 WASHINGTON, D.C. 20549

FORM 8-K
CURRENT REPORT
PURSUANT TO SECTION 13 OR 15(d) OF THE
SECURITIES EXCHANGE ACT OF 1934
Date of Report (Date of earliest event reported): December 3, 2001

Aon CORPORATION
(Exact name of Registrant as Specified in Its Charter)

Delaware
 (State or Other Jurisdiction
of Incorporation)	1-7933 
(Commission File Number)	36-3051915 
(IRS Employer
Identification No.)	

200 East Randolph Street, Chicago, Illinois 
(Address of Principal Executive Offices)	
60601 
(Zip Code)	
Registrant's Telephone Number, Including Area Code: (312) 381-1000
Not Applicable
 (Former Name or Former Address, if Changed Since Last Report)

 Item 7.  Financial Statements, Pro Forma Financial Information and Exhibits.
(a)
Not applicable.

(b)
Pro forma financial information

 Aon Corporation
Pro Forma Condensed Consolidated Financial Information
    Aon Corporation is a holding company that is comprised of a family of insurance brokerage, consulting and insurance underwriting subsidiaries. Through our insurance brokerage and other services and consulting operations, we offer commercial insurance brokerage, alternative risk solutions, risk management, employee benefit and human resources consulting and managing general underwriting services. In addition, our insurance underwriting businesses provide a variety of insurance products, including supplemental accident and health coverage, traditional life insurance and extended warranties. Our revenues were $7.4 billion in 2000. Based on 2000 insurance brokerage and consulting revenues, we are the second largest insurance brokerage company in the world.
    On April 20, 2001, our Board of Directors approved, in principle, a plan to spin-off our underwriting businesses to our common stockholders, creating two independent, publicly traded companies. The spin-off would take the form of a tax-free dividend of the outstanding shares of common stock of Combined Specialty Corporation (CSC), a new company formed to hold our underwriting businesses. The transaction requires final board approval, a favorable ruling from the Internal Revenue Service and certain insurance regulatory approvals, and it is currently expected to be completed in Spring 2002.
    The underwriting businesses we plan to spin-off are mainly reflected in our insurance underwriting segment, one of our three operating segments. This segment currently provides supplemental accident and health and life insurance and extended warranty and property and casualty insurance products through five major subsidiaries: Combined Insurance Company of America, Combined Life Insurance Company of New York, Virginia Surety Company, Inc., London General Insurance Company Limited and Aon Warranty Group, Inc. These subsidiaries hold virtually all of our fixed maturity investments (principally reflected in our insurance underwriting segment) and equity security investments and limited partnership interests (primarily reflected in our corporate and other segment).
    The Pro Forma Condensed Consolidated Statements of Income for the nine months ended September 30, 2001 and the year ended December 31, 2000 are presented on a basis as if the spin-off occurred on January 1, 2000. The Pro Forma Condensed Consolidated Statement of Financial Position as of September 30, 2001 is presented on a basis of recording the effect of the spin-off as of September 30, 2001. The Pro Forma Condensed Consolidated Financial Information is based on various assumptions made by Aon's management as of the current date, including assumptions regarding allocations of assets and liabilities that are expected to be transferred to CSC prior to the spin-off. The actual basis on which the spin-off is effected may vary materially from the assumptions based upon the terms of any final Internal Revenue Service approval, Board of Directors approval, regulatory approval and other factors. The Pro Forma Condensed Consolidated Financial Information does not purport to be indicative of the financial condition or results of operations that would actually have been reported had the spin-off been completed on the date indicated or that may be reported in the future. It also does not reflect the effect of any possible financings. Except for the Aon historical condensed consolidated statement of income data for the year ended December 31, 2000, which was derived from Aon's audited historical financial statements, the Pro Forma Condensed Consolidated Financial Information is unaudited.
    This Pro Forma Condensed Consolidated Financial Information does not include certain disclosures required in accordance with accounting principles generally accepted in the United States and, therefore, should be read in conjunction with Aon's historical financial statements and notes thereto.
Dated: December 3, 2001
2

 Aon Corporation
Pro Forma Condensed Consolidated Statement of Income
(millions, except per share data)

 	 	Nine Months Ended September 30, 2001
	 	
 	 	 	 	Pro Forma Adjustments
	 	 	 	
 	 	Historical Aon
	 	Less CSC
	 	Plus Adjustments
	 	Pro Forma Aon
	 	
Revenue	 	 	 	 	 	 	 	 	 	 	 	 	 	
 	Brokerage commissions and fees	 	$	3,925	 	$	182	 	$	—	 	$	3,743	 	
 	Premiums and other	 	 	1,510	 	 	1,510	 	 	 	 	 	—	 	
 	Investment income (1)(2)(3)	 	 	205	 	 	95	 	 	9	 	 	119	 	
 	 		 		 		 		 	
 	 	Total revenue	 	 	5,640	 	 	1,787	 	 	9	 	 	3,862	 	

Expenses	

 	

 	

 	

 	
 	
 	General expenses	 	 	4,303	 	 	848	 	 	 	 	 	3,455	 	
 	Benefits to policyholders	 	 	823	 	 	823	 	 	 	 	 	—	 	
 	Interest expense (3)(4)	 	 	98	 	 	32	 	 	9	 	 	75	 	
 	Amortization of intangible assets	 	 	118	 	 	6	 	 	 	 	 	112	 	
 	Unusual expenses—World Trade Center	 	 	53	 	 	45	 	 	 	 	 	8	 	
 	 		 		 		 		 	
 	 	Total expenses	 	 	5,395	 	 	1,754	 	 	9	 	 	3,650	 	
Income Before Income Tax and Minority Interest	 	 	245	 	 	33	 	 	 	 	 	212	 	
 	Provision for income tax	 	 	95	 	 	12	 	 	 	 	 	83	 	
 	 		 		 		 		 	
Income Before Minority Interest	 	 	150	 	 	21	 	 	 	 	 	129	 	
 	Minority interest—8.205% trust preferred capital
securities	 	 	(30	)	 	 	 	 	 	 	 	(30	)	
 	 		 		 		 		 	
Income from Continuing Operations	 	$	120	 	$	21	 	$	—	 	$	99	 	

Average Shares Outstanding—Basic	

268	

 	

 	

268	
 	
Average Shares Outstanding—Fully Diluted	 	 	271	 	 	 	 	 	 	 	 	271	 	

Basic Income from Continuing Operations Per Share	
 	
$	
0.44	

 	

 	
 	
$	
0.36	

Fully Diluted Income from Continuing Operations Per Share	
 	
$	
0.44	

 	

 	
 	
$	
0.36	

(1)
Historical Aon investment income by segment is as follows:

 	Insurance brokerage and other services	 	$	127	 	 	 	 	 	 	
 	Consulting	 	 	4	 	 	 	 	 	 	
 	Insurance underwriting	 	 	170	 	 	 	 	 	 	
 	Corporate and other	 	 	(96	)	 	 	 	 	 	
 	 		 	 	 	 	 	 	
 	 	Total	 	$	205	 	 	 	 	 	 	
 	 		 	 	 	 	 	 	
(2)
CSC investment income includes a loss of $56 million for the change in valuation on private limited partnership investments.

(3)
Investment income and interest expense have been increased by $9 million to reflect interest on debt owed to CSC by Aon which was previously eliminated in consolidation.

(4)
As part of the spin-off, it is anticipated that CSC will effectively be allocated debt from Aon. The interest on this allocation is $32 million for the nine months ended September 30, 2001.
Dated: December 3, 2001
3

 Aon Corporation
Pro Forma Condensed Consolidated Statement of Income
(millions, except per share data)

 	 	Year Ended December 31, 2000
	 	
 	 	 	 	Pro Forma Adjustments
	 	 	 	
 	 	Historical Aon
	 	Less CSC
	 	Plus Adjustments
	 	Pro Forma Aon
	 	
Revenue	 	 	 	 	 	 	 	 	 	 	 	 	 	
 	Brokerage commissions and fees	 	$	4,946	 	$	229	 	$	—	 	$	4,717	 	
 	Premiums and other	 	 	1,921	 	 	1,921	 	 	 	 	 	—	 	
 	Investment income(1)(2)(3)	 	 	508	 	 	340	 	 	20	 	 	188	 	
 	 		 		 		 		 	
 	 	Total revenue	 	 	7,375	 	 	2,490	 	 	20	 	 	4,905	 	
Expenses	 	 	 	 	 	 	 	 	 	 	 	 	 	
 	General expenses	 	 	5,190	 	 	1,060	 	 	 	 	 	4,130	 	
 	Benefits to policyholders	 	 	1,037	 	 	1,037	 	 	 	 	 	—	 	
 	Interest expense(3)(4)	 	 	140	 	 	42	 	 	20	 	 	118	 	
 	Amortization of intangible assets	 	 	154	 	 	9	 	 	 	 	 	145	 	
 	 		 		 		 		 	
 	 	Total expenses	 	 	6,521	 	 	2,148	 	 	20	 	 	4,393	 	
Income Before Income Tax, Minority Interest and Accounting Change	 	 	854	 	 	342	 	 	 	 	 	512	 	
 	Provision for income tax	 	 	333	 	 	127	 	 	 	 	 	206	 	
 	 		 		 		 		 	
Income Before Minority Interest and Accounting Change	 	 	521	 	 	215	 	 	 	 	 	306	 	
 	Minority interest—8.205% trust preferred capital securities	 	 	(40	)	 	 	 	 	 	 	 	(40	)	
 	 		 		 		 		 	
Income from Continuing Operations Before Accounting Change	 	$	481	 	$	215	 	$	—	 	$	266	 	
 	 		 		 		 		 	
Average Shares Outstanding—Basic	 	 	260	 	 	 	 	 	 	 	 	260	 	
Average Shares Outstanding—Fully Diluted	 	 	263	 	 	 	 	 	 	 	 	263	 	
Basic Income from Continuing Operations Before Accounting Change Per Share	 	$	1.84	 	 	 	 	 	 	 	$	1.01	 	
 	 		 	 	 	 	 	 	 		 	
Fully Diluted Income from Continuing Operations Before Accounting Change Per Share	 	$	1.82	 	 	 	 	 	 	 	$	1.00	 	

(1)
Historical Aon investment income by segment is as follows:

Insurance brokerage and other services	
 	
$	
186	

 	

 	
 	Consulting	 	 	6	 	 	 	 	 	 	
 	Insurance underwriting	 	 	245	 	 	 	 	 	 	
 	Corporate and other	 	 	71	 	 	 	 	 	 	
 	 		 	 	 	 	 	 	
 	 	Total	 	$	508	 	 	 	 	 	 	
 	 		 	 	 	 	 	 	
(2)
CSC investment income includes revenue of $73 million for the change in valuation on private limited partnership investments.
(3)
Investment income and interest expense have been increased by $20 million to reflect interest on debt owed to CSC by Aon which was previously eliminated in consolidation.
(4)
As part of the spin-off, it is anticipated that CSC will effectively be allocated debt from Aon. The interest on this allocation is $42 million for the year ended December 31, 2000.

Dated: December 3, 2001
4

 Aon Corporation
Pro Forma Condensed Consolidated Statement of Financial Position
(millions)

 	 	As of September 30, 2001
	 	
 	 	 	 	Pro Forma Adjustments
	 	 	 	
 	 	Historical Aon
	 	Less CSC
	 	Plus Adjustments
	 	Pro Forma Aon
	 	
Assets:	 	 	 	 	 	 	 	 	 	 	 	 	 	
 	Fixed maturities at fair value(1)	 	$	2,336	 	$	2,511	 	$	197	 	$	22	 	
 	Equity securities at fair value	 	 	416	 	 	400	 	 	 	 	 	16	 	
 	Short-term investments	 	 	2,899	 	 	645	 	 	 	 	 	2,254	 	
 	Other investments	 	 	736	 	 	713	 	 	 	 	 	23	 	
 	 		 		 		 		 	
 	 	Total Investments	 	 	6,387	 	 	4,269	 	 	197	 	 	2,315	 	
 	Cash	 	 	498	 	 	79	 	 	 	 	 	419	 	
 	Receivables	 	 	8,353	 	 	1,084	 	 	 	 	 	7,269	 	
 	Intangible assets	 	 	4,095	 	 	281	 	 	 	 	 	3,814	 	
 	Other assets	 	 	3,512	 	 	1,702	 	 	 	 	 	1,810	 	
 	 		 		 		 		 	
 	 	 	TOTAL ASSETS	 	$	22,845	 	$	7,415	 	$	197	 	$	15,627	 	
 	 		 		 		 		 	
Liabilities and Stockholders' Equity:	 	 	 	 	 	 	 	 	 	 	 	 	 	
 	Insurance premiums payable	 	$	8,722	 	$	7	 	$	—	 	$	8,715	 	
 	Policy liabilities	 	 	5,034	 	 	5,034	 	 	 	 	 	—	 	
 	General liabilities	 	 	 	 	 	 	 	 	 	 	 	 	 	
 	 	General expenses	 	 	1,685	 	 	137	 	 	 	 	 	1,548	 	
 	 	Short-term borrowings	 	 	193	 	 	 	 	 	 	 	 	193	 	
 	 	Notes payable(1)(2)	 	 	1,660	 	 	600	 	 	197	 	 	1,257	 	
 	 	Other liabilities	 	 	1,075	 	 	548	 	 	 	 	 	527	 	
 	 		 		 		 		 	
 	 	 	TOTAL LIABILITIES	 	 	18,369	 	 	6,326	 	 	197	 	 	12,240	 	
 	Redeemable preferred stock	 	 	50	 	 	 	 	 	 	 	 	50	 	
 	Company-obligated mandatorily redeemable preferred capital securities of subsidiary trust holding solely the company's junior subordinated debentures	 	 	800	 	 	 	 	 	 	 	 	800	 	
 	Stockholders' equity	 	 	 	 	 	 	 	 	 	 	 	 	 	
 	 	Common stock	 	 	292	 	 	 	 	 	 	 	 	292	 	
 	 	Paid-in-capital	 	 	1,650	 	 	 	 	 	 	 	 	1,650	 	
 	 	Accumulated other comprehensive loss	 	 	(377	)	 	(122	)	 	 	 	 	(255	)	
 	 	Retained earnings	 	 	3,060	 	 	1,211	 	 	 	 	 	1,849	 	
 	 	Less: Treasury stock at cost	 	 	(807	)	 	 	 	 	 	 	 	(807	)	
 	 	 	 	Deferred compensation	 	 	(192	)	 	 	 	 	 	 	 	(192	)	
 	 		 		 		 		 	
 	 	 	TOTAL STOCKHOLDERS' EQUITY	 	 	3,626	 	 	1,089	 	 	 	 	 	2,537	 	
 	 		 		 		 		 	
 	 	 	TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	 	$	22,845	 	$	7,415	 	$	197	 	$	15,627	 	

(1)
Fixed maturities and notes payable have been increased by $197 million to reflect debt owed to CSC by Aon which was previously eliminated in consolidation.
(2)
As part of the spin-off, it is anticipated that CSC will effectively be allocated $600 million of debt from Aon.
Dated: December 3, 2001
(c)
Exhibits.
    None.
5

 SIGNATURES
    Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

 	 	AON CORPORATION	

 	
By:	
 	
/s/  RAYMOND I. SKILLING    

Raymond I. Skilling
 Executive Vice President and Chief Counsel	
Date: December 3, 2001
6

QuickLinks
Item 7. Financial Statements, Pro Forma Financial Information and Exhibits.
Pro Forma Condensed Consolidated Financial Information
 Pro Forma Condensed Consolidated Statement of Income (Nine Months Ended September 30, 2001)
 Pro Forma Condensed Consolidated Statement of Income (Year Ended December 31, 2000)
Pro Forma Condensed Consolidated Statement of Financial Position
 SIGNATURES