Abstract We have used isotope transients, 16O 2– 18O 2, at reaction steady state to investigate the epoxidation of ethylene over unsupported silver powder catalysts. The reactive oxygen inventory at steady state consists of two types: one with a short residence time (<1s on the catalyst) and another with a long residence time (>15s). The latter (assigned to subsurface oxygen) can comprise as much as 2.5 monolayer-equivalents. Ethylene oxide selectivity correlates strongly with the availability of the subsurface oxygen. This correlation holds for both chlorine-free and chlorine-inhibited catalysts and over a range of temperatures.