<Page> Exhibit 12(a) AON CORPORATION AND CONSOLIDATED SUBSIDIARIES COMBINED WITH UNCONSOLIDATED SUBSIDIARIES COMPUTATION OF RATIO OF EARNINGS TO FIXED CHARGES <Table> <Caption> NINE MONTHS ENDED SEPTEMBER 30, YEARS ENDED DECEMBER 31, --------------------- ----------------------------------------------------- (millions except ratios) 2002 2001 (1) 2001 2000 1999 1998 1997 -------- ----------- -------- ---------- ----------- ---------- -------- (RESTATED) Income before provision for income taxes and minority interest $ 504 $ 245 $ 309 $ 854 $ 635 $ 931 $ 542 ADD BACK FIXED CHARGES: Interest on indebtedness 91 98 127 140 105 87 70 Interest credited on investment-type insurance contracts 27 44 56 71 77 72 45 Interest on ESOP - - - - 1 2 3 Portion of rents representative of interest factor 38 39 57 54 49 51 44 -------- ----------- -------- ---------- ----------- ---------- -------- INCOME AS ADJUSTED $ 660 $ 426 $ 549 $ 1,119 $ 867 $ 1,143 $ 704 ======== =========== ======== ========== =========== ========== ========== FIXED CHARGES: Interest on indebtedness $ 91 $ 98 $ 127 $ 140 $ 105 $ 87 $ 70 Interest credited on investment-type insurance contracts 27 44 56 71 77 72 45 Interest on ESOP - - - - 1 2 3 Portion of rents representative of interest factor 38 39 57 54 49 51 44 -------- ----------- -------- ---------- ----------- ---------- -------- TOTAL FIXED CHARGES $ 156 $ 181 $ 240 $ 265 $ 232 $ 212 $ 162 ======== =========== ======== ========== =========== ========== ========== RATIO OF EARNINGS TO FIXED CHARGES 4.2 2.4 2.3 4.2 3.7 5.4 4.3 ======== =========== ======== ========== =========== ========== ========== </Table> - -------------------- (1) The computation of Ratio of Earnings to Fixed Charges for nine months ended September 30, 2001 has been restated from what had previously been reported to include interest credited on investment-type insurance contracts.