A series of aromatic polyimides was prepared via two-step polymerization and subsequent thermal imidization to manufacture carbon molecular sieve (CMS) membranes. In order to know the effect of microstructural changes of the polyimides on the gas permeation proper-ties of their CMS membranes, three copolyimides were designed and synthesized using a dianhydride and diamines with a different number of methyl substituent groups. The density, fractional free volume (FFV), and glassy transition temperature (T-g) of the polyimides were characterized to investigate the microstructural changes of the polyimides which led to the difference in gas permeation properties as well as that in their physical properties. The introduction of methyl substituent group in the rigid polyimide backbone increased FFV of polyimides, and the gas permeabilities increased typically with FFV. After an inert pyrolysis of these polyimides, similar gas permeation behaviors were also observed in their CMS membranes pyrolyzed at 600 and 800degreesC, respectively. That is, the gas permeation characteristics of the polyimide due to the structural change were well kept even after pyrolysis within the temperature range used in this work. In all cases, the gas permeabilities increased in the order He > CO2 > O-2 > N-2, indicating that the gas permeabilities are in agreement with the order of the kinetic gas diameters. The CMS membranes prepared in this study exhibited good separation properties on He/N-2, O-2/N-2, and CO2/N-2 with satisfactory gas permeabilities. Thereby, the present work will provide an insight into the molecular design of the polymeric precursors for the effective preparation of the CMS membranes. (C) 2003 Elsevier B.V. All rights reserved.