We have used isotope transients, O-16(2)-O-18(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.