The process of concentrating low-penetrating impurities in membrane modules was studied theoretically and experimentally. An expression was derived for the degree of separation in a radial cross-flow module; the degree of separation substantially depends on the gas-phase diffusion coefficient of the impurity. The experimental results obtained for a mixture of carbon dioxide with an impurity of Freon 14 and a mixture of Freon 12 with an impurity of Freon 218 agree with calculated data.