Comparative data is reported for the conversion of ethylene to ethylene oxide over silver catalysts using oxygen and nitrous oxide as oxidants. For unsupported silver sponge and several supported silver catalysts the selectivity to ethylene oxide when using oxygen is ca.45% between 170 and 250°C. With nitrous oxide, the selectivity is 24% at 190°C declining to 14% at 240°C. Activation energies for conversion of ethylene to all products is similar for the two oxidants but the rate is approximately six times higher with oxygen than nitrous oxide. The pressure dependencies are very similar for the two reactions. Using partial pressures from 10 to 200 Torr, there is a minimum in respect of ethylene pressure but not for oxidant pressure. Variations in selectivity with pressure are slight. When dichloroethane (< 6 ppm) is included in the feed, the rate declines and selectivity rises. With oxygen as oxidant a steady state is reached when the ratio of chlorine atoms fed to surface silver atoms reaches ca. 0.2. The selectivity is then ca. 80% and the steady state rate is inversely related to the dichloroethane concentration. Under the same conditions with nitrous oxide as oxidant, the rate declines to undetectable levels and the highest selectivity measured prior to rate extinction is 31%. Removal of dichloroethane from the feed is followed by a slow recovery in rate when oxygen is used but no recovery when using nitrous oxide. Reduction of the chlorine moderated catalyst with hydrogen restores the original catalyst properties for both reactions. The implications of these findings in respect of the likely mechanisms is discussed.