QuickLinks -- Click here to rapidly navigate through this document

Exhibit 12(a)

Aon Corporation and Consolidated Subsidiaries
Combined With Unconsolidated Subsidiaries
Computation of Ratio of Earnings to Fixed Charges

			Six Months Ended June 30,				Years Ended December 31,
			2002		2001		2001		2000		1999		1998		1997
					(Restated)		(Restated)		(Restated)		(Restated)		(Restated)		(Restated)
			(millions except ratios)
Income before provision for income taxes and minority interest			$	287	$	112	$	309	$	854	$	635	$	931	$	542
Add back fixed charges:
	Interest on indebtedness			59		67		127		140		105		87		70
	Interest credited on investment-type insurance contracts			21		30		56		71		77		72		45
	Interest on ESOP			—		—		—		—		1		2		3
	Portion of rents representative of interest factor			28		27		57		54		49		51		44
		Income as adjusted	$	395	$	236	$	549	$	1,119	$	867	$	1,143	$	704
Fixed charges:
	Interest on indebtedness		$	59	$	67	$	127	$	140	$	105	$	87	$	70
	Interest credited on investment-type insurance contracts			21		30		56		71		77		72		45
	Interest on ESOP			—		—		—		—		1		2		3
	Portion of rents representative of interest factor			28		27		57		54		49		51		44
		Total fixed charges	$	108	$	124	$	240	$	265	$	232	$	212	$	162
Ratio of earnings to fixed charges				3.7		1.9		2.3		4.2		3.7		5.4		4.3

QuickLinks

Aon Corporation and Consolidated Subsidiaries Combined With Unconsolidated Subsidiaries Computation of Ratio of Earnings to Fixed Charges