Carbon molecular sieve (CMS) membranes were prepared using aromatic polyimide (PI) and polyvinylpyrrolidone (PVP) polymer blends. The characteristics of the carbon structures and the gas permeation properties of the CMS membranes pyrolyzed at 500 and 700 degrees C were investigated in terms of the molecular weight of PVP (the thermally labile polymer). The PI/PVP blend showed two separate thermal decomposition steps. In an argon atmosphere, PVP underwent complete decomposition first at 350-450 degrees C, while the thermal degradation of PI occurred at 550-650 degrees C. The nitrogen adsorption isotherm of the CMS membranes showed that the thermal decomposition of PVP significantly affected the average pore volume of the resultant CMS membranes, and that it depended upon the molecular weight of PVP. CMS membranes with a higher molecular weight of PVP pyrolyzed at 550 degrees C exhibited an enhanced 02 gas permeability from 4200.28 x 10(-18) to 6075.405 x 10(-18) m(2) s(-1) Pa-1 (560 to 810 barrers) and a reduced O-2/N-2 selectivity of 10-7. (c) 2004 Elsevier BN. All rights reserved.