The separation of a gas from a two component gas system was studied both theoretically and experimentally. Helium and argon as an inorganic gas pair, ethylene and butane as an organic gas pair, and air were used. The theory based on the free molecular flow could not be applied to all the combinations of the gases experimentally studied. Experiments showed that the permeability coefficient of each component in the mixture was very close to that of each single component, when the pressure in the low pressure side became zero, but those of the mixture approached one another rapidly as the pressure increased. In the case of the separation of one inorganic gas from another, there exists most suitable pressures both in the high and low pressure sides for the most efficient separation, as considered from the selectivity and permeability. In the cases of the mixed organic gases and the organic-inorganic gas combination, a rather high separation efficiency is expected for a very high pressure on the high pressure side.