Measurements are reported of effective diffusivity of oxygen in nitrogen through the pores of various graphites with widely different permeabilities. The effect of back purging with nitrogen was studied. A thousandfold reduction of apparent diffusivity occurred with back pressures ranging from about 3 to 720 torr according to the type of graphite. The permeability constants ( B 0 and K 0 ) were measured and introduced into a theoretical relationship which predicted the back diffusion data. The effect of pore structure on back diffusion and the application of the results to a type of high temperature, gas-cooled reactor are discussed.