Several binary gas mixtures have been fractionated by diffusion through the high surface area microporous carbon plug characterized in part I (Ash, Barrer & Pope, previous paper). The efficiency of the separation processes varied with the differences in condensability of the pairs of gases and with their partial pressures. The dominant mode of transport of the less volatile components was on the surface of the adsorbent, so that the fractionation factors bore no relation to the rates of diffusion in the gas phase. The blockage of gas-phase flow by the adsorbed phase has been investigated quantitatively. This blockage can be so effective that the membrane becomes impermeable to a non-sorbed component (H$_2$) while being freely permeable to a strongly sorbed component (SO$_2$). Very high fractionation factors were also obtained with N$_2$ + CO$_2$ and Ne + CO$_2$ mixtures. A + N$_2$ mixtures showed a much smaller fractionation, both components being adsorbed to a comparable extent.