The oxidation of ethylene and propylene labeled with deuterium in various positions and the cooxidation of unlabeled olefins with the corresponding 14C-labeled epoxides have been studied over a silver catalyst. The latter measurements showed that, in both systems at 200 to 220 °C, a portion of the CO2 was produced by destruction of the product epoxide, but that the oxygen exchange between olefin and epoxide was nil. Oxidation of either cis- or trans-ethylene-d2 yielded a mixture of the cis- and trans-d2 epoxides which was about 92% equilibrated. Relative rate measurements showed that substitution of deuterium for hydrogen in ethylene increased the yield of epoxide substantially. Such kinetic isotope effects were even more pronounced with propylene for which the oxidation of CD3CHCH2 and CD3CDCD2 gave a 10 to 14% selectivity to the corresponding epoxides compared with 2 to 5% for CH3CHCH2, CH3CDCH2, and CH3CHCD2. The kinetic isotope effects can be qualitatively explained in terms of a normal primary effect in the further oxidation of an intermediate which is common to both epoxide formation and total oxidation. The significance of these findings to previous suggestions regarding surface intermediates and the mechanism of these oxidations is discussed.