QuickLinks -- Click here to rapidly navigate through this document

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): April 5, 2002

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

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

200 East Randolph Street, Chicago, Illinois
 
60601
(Address of Principal Executive Offices)   (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 capital consulting and underwriting management services. In addition, our insurance underwriting businesses provide a variety of insurance products, including supplemental accident and health coverage, extended warranties and specialty property and casualty products. Our revenues were $7.7 billion in 2001. Based on 2001 revenues, we are the second largest insurance brokerage company in the world.

        On April 20, 2001, Aon's Board of Directors approved, in principle, a plan to spin-off our insurance underwriting businesses to Aon's common stockholders, creating two independent publicly traded companies. The spin-off would take the form of a tax-free stock dividend of the outstanding shares of common stock of Combined Specialty Group, Inc. (CSG), formerly known as Combined Specialty Corporation, a new company formed to hold our underwriting businesses. The transaction requires final Board approval, a favorable Internal Revenue Service tax ruling and certain insurance regulatory approvals, and 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, life insurance, extended warranty and specialty property and casualty insurance products through five major subsidiaries: Combined Insurance Company of America, Combined Life Insurance Company of New York, Combined Specialty Insurance Company, 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 segment). On December 31, 2001, Aon securitized $450 million of its limited partnership investments plus associated limited partnership commitments, which represented the majority of its limited partnership interests. Aon received a combination of cash and securities in connection with the securitization. In 2001, the underwriting businesses invested $227 million to obtain an ownership interest in Endurance Specialty Insurance Ltd. (Endurance), a newly formed Bermuda-based insurer, which will offer property and casualty insurance and reinsurance on a worldwide basis.

2


Aon Corporation

Pro Forma Condensed Consolidated Financial Information

        Attached hereto is a Pro Forma Condensed Consolidated Statement of Income for the year ended December 31, 2001 on a basis as if the spin-off occurred on January 1, 2001. Also attached is a Pro Forma Condensed Consolidated Statement of Financial Position as of December 31, 2001, presented on a basis of recording the effect of the spin-off as of December 31, 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 CSG prior to the spin-off. The actual basis on which the spin-off is effected may vary 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 does not reflect the effect of any possible financings. The historical Aon and the CSG financial information has been derived from the audited financial statements of the respective companies. The adjustments made to the historical financial information of Aon and CSG are 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 and CSG's respective historical financial statements and notes thereto. This information should also be read in conjunction with Aon's and CSG's respective management's discussion and analysis of financial condition and results of operations as reflected in recent Securities and Exchange Commission filings.

Dated: April 5, 2002

3

Aon Corporation
Pro Forma Condensed Consolidated Statement of Income

 
   Year ended December 31, 2001
  
 
    
   Pro Forma Adjustments
    
  
 
   Historical
Aon
   Less
CSG
   Plus
Adjustments
   Pro
Forma
Aon
  
 
   (millions, except per share data)

  
Revenue                          
  Brokerage commissions and fees (5)   $ 5,436   $ 213   $ (52 ) $ 5,171  
  Premiums and other     2,027     2,027           —  
  Investment income (1,2,4)     213     88     20     145  
   
 
 
 
 
    Total revenue     7,676     2,328     (32 )   5,316  
Expenses                          
  General expenses (5)     5,813     1,091     (52 )   4,670  
  Benefits to policyholders     1,111     1,111           —  
  Interest expense (3,4)     127     42     20     105  
  Amortization of intangible assets     158     10           148  
  Unusual charges—World Trade Center     68     45           23  
   
 
 
 
 
    Total expenses     7,277     2,299     (32 )   4,946  

Income Before Income Tax and Minority Interest
 
 
399
 
 
29
 
 
 
 
 
370
 
  Provision for income tax     156     11           145  
   
 
 
 
 
Income Before Minority Interest     243     18           225  
  Minority interest—8.205% trust preferred capital securities     (40 )               (40 )
   
 
 
 
 
Net Income   $ 203   $ 18   $ —   $ 185  
   
 
 
 
 
Net Income Available for Common Stockholders   $ 200               $ 182  
   
             
 
Average Shares Outstanding—Basic     269                 269  
Average Shares Outstanding—Dilutive     272                 272  

Basic Net Income Per Share
 
$
0.74
 
 
 
 
 
 
 
$
0.68
 
   
             
 
Dilutive Net Income Per Share:                          
  Income before unusual and special charges   $ 1.37               $ 1.15  
  Unusual charges—World Trade Center     (0.15 )               (0.05 )
  Special charges     (0.49 )               (0.43 )
   
             
 
    Net Income   $ 0.73               $ 0.67  
   
             
 
 
    
    
    
    
Notes:                  
1.    Historical Aon investment income by segment is as follows:            
  Insurance brokerage and other services   $ 156            
  Consulting     5            
  Insurance underwriting     223            
  Corporate and other     (171 )          
   
           
    Total   $ 213            
   
           
2.
CSG investment income includes a loss of $94 million for the change in valuation on private limited partnership investments. Reference should be made to comments on the previous page regarding the limited partnership securitization and investment in Endurance.

4

3.
As part of the spin-off, it is anticipated that CSG will effectively be allocated debt from Aon. The interest on this allocation is $42 million for the twelve months ended December 31, 2001.

4.
Investment income and interest expense have been increased by $20 million to reflect interest on debt owed to CSG by Aon which was previously eliminated in consolidation.

5.
Brokerage commissions and fees and general expenses have been decreased by $52 million to adjust for certain amounts reclassified for CSG's separate company financial statements.

Dated: April 5, 2002

5


Aon Corporation
Pro Forma Condensed Consolidated Statement of Financial Position

 
   As of December 31, 2001
  
 
    
   Pro Forma Adjustments
    
  
 
   Historical
Aon
   Less
CSG
   Plus
Adjustments
   Pro
Forma
Aon
  
 
   (millions)

  
Assets:                          
  Fixed maturities at fair value (1)   $ 2,149   $ 2,425   $ 313   $ 37  
  Equity securities at fair value     382     363           19  
  Short-term investments     2,975     696           2,279  
  Other investments     640     605           35  
   
 
 
 
 
      Total Investments     6,146     4,089     313     2,370  
  Cash     439     82           357  
  Receivables     7,986     867           7,119  
  Intangible assets     4,084     317           3,767  
  Other assets     3,731     1,964           1,767  
   
 
 
 
 
  TOTAL ASSETS   $ 22,386   $ 7,319   $ 313   $ 15,380  
   
 
 
 
 
Liabilities and Stockholders' Equity:                          
  Insurance premiums payable   $ 8,233   $ —   $ —   $ 8,233  
  Policy liabilities     4,990     4,990           —  
  General liabilities                          
    General expenses     1,770     140           1,630  
    Short-term borrowings     257                 257  
    Notes payable (1,2)     1,694     600     313     1,407  
    Other liabilities     1,071     413           658  
   
 
 
 
 
    TOTAL LIABILITIES     18,015     6,143     313     12,185  
  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     293                 293  
    Paid-in-capital     1,654                 1,654  
    Accumulated other comprehensive loss     (535 )   (135 )         (400 )
    Retained earnings     3,077     1,311           1,766  
    Less: Treasury stock at cost     (786 )               (786 )
              Deferred compensation     (182 )               (182 )
   
 
 
 
 
    TOTAL STOCKHOLDERS' EQUITY     3,521     1,176     —     2,345  
   
 
 
 
 
  TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY   $ 22,386   $ 7,319   $ 313   $ 15,380  
   
 
 
 
 

Notes:

1.
Fixed maturities and notes payable have been increased by $313 million to reflect debt owed to CSG by Aon which was previously eliminated in consolidation.

2.
As part of the spin-off, it is anticipated that CSG will effectively be allocated $600 million of debt from Aon.

Dated: April 5, 2002

(c)
Exhibits.

6

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: April 5, 2002




QuickLinks