The mechanism of ethylene epoxidation over a silver sponge catalyst under steady-state conditions has been probed by comparison of nitrous oxide and oxygen as oxidants. Under noncompetitive conditions the reaction with oxygen is about five times as fast and more than twice as selective. When both are present oxygen inhibits participation by nitrous oxide and the ratio of rates is much larger. Reaction has been carried out with a mixture of 18O2 and N2 16O. The aim was to see if 18O was preferentially incorporated into ethylene and 16O into carbon dioxide, as would be expected if diatomic adsorbed oxygen is the source of the epoxide. The data indicate that this does not occur and hence that ethylene oxide and carbon dioxide are both derived from oxygen atoms. However, it is very difficult to exclude artifacts since the minimal involvement of nitrous oxide makes the isotope analyses difficult. No isotopic mixing occurred between 18O2 and N2 16O; neither is it observable when ethylene is oxidized by 16O2/18O2 mixtures. It is detectable when ethylene is absent but the rate is lower than that of ethylene oxidation under similar conditions. Studies have been made using the same silver catalyst promoted by inclusion of dichloroethane in the feed. This raises selectivity to 80% when oxygen is used but the rate is much lower. The chlorine-moderated catalysts have no measurable activity for ethylene oxidation by nitrous oxide. This difference in behavior between the two oxidants can be correlated with their dissociation rates in the absence of ethylene. Surface chlorine sufficient to depress isotope mixing between 16O2 and 18O2 by a factor of 7 relative to unpromoted silver renders dissociation of nitrous oxide undetectable. Dissociation of nitrous oxide is also inhibited by adsorbed oxygen even when surface chlorine is present. It is concluded that the rate of ethylene oxidation is related to the rates of dissociation of the respective oxidants. Ethylene oxidation can be faster than oxidant dissociation in the absence of ethylene because reaction with ethylene prevents buildup of inhibiting surface oxygen atoms.