Quenching of singlet oxygen luminescence at 1.27 mu m by PhS., PhSO., and peroxyl radicals phOO(.), t-BuOO., PhCH2OO., Ph2CHOO., and Ph3COO. was studied in liquid solution. The quantum yields of decomposition of different initiators which lead to the formation of free radicals were measured by using nanosecond transient absorption. This allowed determination of singlet oxygen O-2((1) Delta g) quenching rate constants by the radicals. They are <2 x 10(8) M-1 s(-1) for the sulfur-centered radicals and (2-7) x 10(9) M-1 s(-1) for peroxyl radicals in acetonitrile. The rapid quenching is attributed to energy transfer quenching by the peroxyls, which have an n --> pi* transition leading to a low-lying (2)A' state above their (2)A " ground state. PhSO. is shown computationally not to have such a low-lying 2A' state. There may be a very low-lying B-2(1) state, forPhS(.), but it is apparently not an efficient acceptor of electronic energy from O-2((1) Delta g).