We have measured the extent of 18O incorporation into the products of metal complex-catalyzed oxygenations of organic compounds when H218O is added to the reaction mixture. The oxidants studied were hydrogen peroxide, tert-butyl hydroperoxide, m-chloroperbenzoic acid (MCPBA), and iodosylbenzene, and the reactions were carried out in organic solvents. In reactions of hydrogen peroxide, tert-butyl hydroperoxide, and MCPBA, no or at most a small amount of 18O was incorporated into the products in either olefin epoxidation or alkane hydroxylation reactions catalyzed by (meso-tetrakis(2,6-dichloro- phenyl)porphinato)iron(III) chloride (Fe(TDCPP)Cl), (meso-tetrakis(2,6-dichlorophenyl)porphinato)manganese(III) chloride (Mn(TDCPP)Cl) with imidazole added, iron(II) cyclam (cyclam = 1,4,8,11-tetraazacyclotetradecane), manganese(II) cyclam, and nickel(II) cyclam. Assuming that high-valent metal oxo intermediates are generated in all of the reactions of iron and manganese porphyrin complexes with the oxidants PhIO, H2O2, tert-butyl hydroperoxide, and MCPBA, we conclude that the high-valent iron oxo and manganese oxo intermediates do not exchange or slowly exchange with labeled H218O during the course of these catalytic oxygenation reactions under our reaction conditions. Several different iron(III) and manganese(III) porphyrin complexes such as Fe(TDCPP)Cl, (meso-tetraphenylporphinato)iron(III) chloride (Fe(TPP)Cl), (meso-tetra- mesitylporphinato)iron(III) chloride (Fe(TMP)Cl), and Mn(TDCPP)Cl were used to catalyze cyclohexene epoxidation by MCPBA at low temperature (−78 °C) in the presence of H218O. The epoxide obtained in the epoxidation of cyclohexene catalyzed by Fe(TDCPP)Cl, Fe(TPP)Cl, Fe(TMP)Cl, and Mn(TDCPP)Cl contained 0%, 4%, 22%, and 0% 18O, respectively. By contrast, in the iodosylbenzene reactions, oxygen from labeled H218O was fully incorporated into products in aprotic and protic solvents in olefin epoxidation and alkane hydroxylation reactions catalyzed by either iron(III) porphyrin, manganese(III) porphyrin, or metallocyclam (M = Fe, Mn, Ni) complexes. Labeled oxygen from H218O was also fully incorporated into cyclohexene oxide in the epoxidation of cyclohexene catalyzed by a zinc complex which is not able to form a high-valent zinc oxo species as an intermediate. We conclude from these results that, in the case of iodosylbenzene, the mechanism for oxygen exchange does not involve metal oxo intermediates and that the observation of incorporation of labeled oxygen from H218O into products does not provide evidence for the intermediacy of metal oxo complexes in iodosylbenzene reactions. In the case of oxidants other than iodosylbenzene, our results also suggest that reactions of high-valent metal oxo complexes with organic substrates in catalytic oxygenation reactions are often comparable in rate to or faster than the reactions with isotopically labeled water that lead to oxygen exchange.