A BPDA-ODA polyimide film was formed on the outer surface of a porous alumina support tube (outside diameter, 2.4 mm; inside diameter, 1.8 mm; void fraction, 0.48; average pore size, 140 nm). It was imidized and then carbonized at 500-900 degrees C. Defects usually generated during carbonization were completely avoided by optimizing temperature profiles. Carbonization above 600 degrees C remarkably increased the permeance of He, CO2, CH4, N-2, and C2H6 without losing the high permselectivity of the polyimide. The ultimate micropore volume determined by CO2 sorption was also increased to 0.36 x 10(-3) from 0.16 x 10(-3) m(3) . kg(-1). However, the micropore volume was not a dominant factor. The increase in permeance, which was mainly controlled by effective diffusivity of permeating species in the membrane, was dependent on pore-size distribution. The CO2 permeance and CO2/CH4 permselectivity of the carbonized membrane were 10(-7) mol . m(-2). s(-1). Pa-1 and 100, respectively, at 30 degrees C.