Carbon molecular-sieve membranes (CMSMs) have shown great potential for gas separation. They exhibit high selectivity by permitting effective size- and shape-separation between gas molecules of similar molecular dimensions. Hence, the control of their pore size is very important. While the previous studies have focused on the conditions of pyrolysis and its effect on CMSM properties, a novel approach is reported here whereby the precursor polymer is chemically modified prior to pyrolysis and the resultant CMSM was investigated for its gas separation properties. Pyrolysis of chemically crosslinked and uncrosslinked Matrimid(R) resulted in a change in d-spacing from 5.6 to 3.2 Angstrom, respectively. The crosslinked CMSM also exhibited greater ordering in its packing. The Matrimid-derived CMSMs exhibited excellent separation properties for CO2/CH4.