Carbon molecular sieve (CMS) membranes were prepared by controlled pyrolysis of polyimide (PI)/polyvinylpyrrolidone (PVP) blends. The average d-spacing values calculated from wide-angle X-ray diffraction data were significantly increased with the amount of PVP. The pyrolytic CMS membranes showed the type I isotherm according to IUPAC classification from the nitrogen adsorption isotherm. From pure gas permeation experiments using He, CO2, O-2, and N-2, it was found that the existence of a thermally labile phase significantly affected the gas permeation performance of the final CMS membranes. The gas permeabilities through the CMS membranes were enhanced by the introduction of thermally labile polymer (PVP) and decreased as the final pyrolysis temperature increased. The CMS membrane derived from PI containing 10wt.% PVP and pyrolyzed at 550 degreesC showed maximum gas permeability for O-2 of 630 Barrers and O-2/N-2 selectivity of 10. For a CMS membrane pyrolyzed at 700 degreesC derived from a precursor containing 10% PVP, the gas permeability for O-2 was 230 Barrers and O-2/N-2, selectivity was 14. (C) 2004 Elsevier B.V. All rights reserved.