Rate constants, kQ, have been determined for the deactivation of singlet oxygen in carbon tetrachloride by a range of transition metal complexes. The singlet oxygen was produced by flash photolysis of air-saturated carbon tetrachloride solutions with methylene blue as sensitizer, and the decay followed by observing the concentration of the accepter 1,3-diphenyliso-benzofuran at 435 nm. The high rate constants of certain diamagnetic nickel complexes are accounted for by a spin-allowed transfer of electronic energy from singlet oxygen to the complex. Rate constants have also been determined for the deactivation of triplet pentacene by a number of these metal complexes. The triplet pentacene was produced by flash photolysis of dilute solutions of pentacene in benzene and the decay monitored at 492 nm. The rate constants for quenching of triplet pentacene are generally lower than those for quenching of singlet oxygen but there is a good correlation between the two sets of results. Both processes appear to proceed by way of electronic energy transfer without the intervention of complex formation between donor and accepter.