A new method in preparing carbon-based molecular sieve (CMS) membranes for gas separation has been proposed. Carbon-based films are deposited on porous Al2O3 disks using hexamethyldisiloxane (HMDSO) by remote inductively coupled plasma (ICP) chemical vapor deposition (CVD). After treating the film with ion bombardment and subsequent pyrolysis at a high temperature, carbon-based molecule sieve membranes can be obtained. exhibiting a very high H-2/N-2, selectivity, around 100 and an extremely high permeance of H-2 around 1.5 x 10(-6) mol m(-2) s(-1) Pa-1 at 298 K. The O-2/N-2 selectivity could reach 5.4 with the O-2 permeance of 2 x 10(-7) mol m(-2) s(-1) Pa-1 at 423 K. During surface treatments, HMDSO ions were found to be more effective than CH4., Ar, O-2 and N-2 ions to improve the selectivity and permeance. Short and optimized surface treatment periods were required for high efficiency. Without pyrolysis. surface treatments alone greatly reduced the H-2 and N-2 permeances and had no effect on the selectivity. Besides, without any surface treatment, pyrolysis alone greatly increased the H-2 and N-2 permeances. but had no improvement on the selectivity, owing to the creation of large pores by desorption of carbon. A combination of surface treatment and pyrolysis is necessary for simultaneously enhancing the permeance and the selectivity of CMS membranes, very different from the conventional pore-plugging mechanism in typical CVD. (C) 2004 Elsevier B.V. All rights reserved.