Quenching of the lowest n,pi* triplet states of various diketones and acetone by molecular oxygen was studied by time-resolved O-1(2) luminescence and nanosecond laser photolysis in solution. The singlet oxygen production efficiency S-Delta in benzene was 0.28 for acetone and 0.28-0.58 for alpha-diketones. The quenching rate constants of alpha-diketone triplets by oxygen (k(q)(T)) are very low, (2-9) x 10(8) M(-1) s(-1). A decrease in O-1(2) lifetime was observed upon excitation of alpha-diketones at laser pulse energies greater than or equal to 1 mJ and is explained by quenching of singlet oxygen by acylperoxy radicals. Rate constants of O-1(2) quenching by different peroxy and acylperoxy radicals k(q) were estimated to be similar to(1-5) x 10(10) M(-1) s(-1), near the diffusion-controlled limit.