Spin-trapping studies of the radicals produced by photolysis of biacetyl in degassed benzene confirmed the formation of acetyl radicals as a major product and methyl radicals as a minor one. In aerated benzene, the trapped radicals are ascribed to the acetylperoxy radical yielded by the reaction of the acetyl radical and oxygen. The laser photolysis studies of biacetyl in aerated benzene containing tetramethyl-p-phenylenediamine, TMPD, demonstrate that the acetylperoxy radicals react with TMPD to give the weakly interacting complex which turns to the ion-pair complex with the rate constant 6.7 x 10(5) s(-1). The absorption spectrum of the ion-pair complex resembles that of the cation radical of TMPD. The quantum yield for the formation of the acetylperoxy radicals produced by photolysis of biacetyl in benzene is estimated as 0.41. The addition of cyclohexene and norbornadiene is found to increase the rate for the formation of the ion-pair complex, indicating that the acetylperoxy radicals react with these olefins : the rate constants are 2.1 x 10(6) M(-1) s(-1) for cyclohexene and 1.8 x 10(7) M(-1) s(-1) for norbornadiene.