Abstract: A gas which is the most readily condensable of the gases of a mixture is separated by passing the mixture in contact with at least one microporous barrier having a permeability of 100.times.10.sup.-7 to 1000.times.10.sup.-7 moles of air/cm.sup.2 /min./cmHg, a pore thickness of 10 to 100 A, by adjusting the partial pressure of the most readily condensable gas and/or the temperature of the gas mixture as a function of the mean pore radius of the barrier in order to ensure capillary condensation of the condensable gas at the pore inlets of the barrier and to induce a flow of condensed gas along the pores and across the barrier, the gas which has thus condensed being then collected.