Asymmetric gas separation membranes, prepared by dry and dry-wet phase inversion, are described. Pure and mixed gas transport properties of the membranes with different thicknesses of the dense skin were determined. The membranes exhibited high permselectivity, further enhanced in time by physical ageing. The polymer was moderately plasticized by CO2 and condensable vapours, reducing the CO2/CH4 selectivity from about 40 for pure gases to about 25 for a 50% CO2/50% CH4 mixture at 10 bar.