A BPDA-pp＇ODA polyimide film was formed on the surface of a porous alumina support tube, which was then carbonized at an optimized temperature of 700 degrees C. The membrane was further oxidized with either an O-2-N-2 mixture Or pure O-2 at 100-300 degrees C. Oxidation at 300 degrees C increased permeances, without damaging permselectivities, of the membrane. No difference in permeances between single and binary CO2-N-2 systems was observed. The carbon membranes possessed slit-shaped molecular sieving pores, in which molecules could pass one another by moving to the wider side of the slit. Permselectivity of the permeating molecules was determined by the narrow width of the slit-shaped micropores.