Carbon molecular sieve (CMS) membranes were prepared from polyimides with sulfonic acid triethylammonium salt (SO3NH(C2H5)(3)) and/or hexafluoroisopropylidene (-C(CF3)(2)-) groups by pyrolyzing precursor composite membranes at the temperatures ranging from 773 to 973 K in an N-2 now for 1 h. The precursor membranes were prepared by coating the polyimides on ceramic porous support tubes. The sulfonic acid salt and the -C(CF3)(2)- group decomposed in the ranges of 573 to 673 K and 723 to 973 K, respectively. The composite CMS membranes prepared here had a defect-free top layer of 1.5 to 5 mu m thick. The CMS membrane derived from the polyimide with both the thermally decomposable groups displayed the highest gas permeances. The decomposition of sulfonic acid groups followed by the decomposition of the -C(CF3)(2)- group just before the substantial decomposition of the polyimide backbone seemed to significantly enhance the micropore structure of the resultant pyrolytic layer. Compared to the CMS composite or hollow-fiber membranes reported in the literature, the membranes prepared in this study displayed higher O-2 and CO2 gas permeances with reasonably high ideal separation factors for O-2/N-2 and CO2/N-2 separations.