The mechanism of the epoxidn. of ethylene with oxygen over silver catalysts has been examd. This study was prompted by the notion that if one wishes to increase the selectivity of epoxide formation one should know the phase of adsorbed oxygen that reacts to give epoxide. Expts. were undertaken (a) to det. whether adsorbed at. oxygen is incorporated into ethylene to give epoxide, and (b) to establish the relative rates of reaction of mol. oxygen impinging from the gas phase and of preadsorbed at. oxygen. Expts. were performed in a high-vacuum recirculation system designed to allow volumetric absorption studies as well as kinetic studies using a mass spectrometer. The catalyst consisted of silver powder precovered with chemisorbed 16O such that it gave epoxide upon reaction with ethylene. It was contacted with ethylene/18O2 gas mixts. of different ratios. It is shown that under conditions where oxygen isotope equilibration in the gas phase is slow, ethylene initially reacts more rapidly with preadsorbed oxygen atoms than with mol. oxygen adsorbed in the precursor state (pO2 <20 Torr; 1 Torr = 133.3 N m-2) if Oads/Ags .apprx. 1. If in the beginning only subsurface oxygen atoms are present, this subsurface oxygen and gas-phase oxygen are incorporated into ethylene at about equal rates. It is concluded that ethylene reacts with at. oxygen to give epoxide either directly, or indirectly, after recombination of the oxygen atoms.