Uncrosslinked and crosslinked polyimides and copolyimides have been synthesized in order to increase selectivity without an unacceptable loss in permeability. The goal was to reduce undesirable effects caused by CO2 induced swelling in CO2/CH4 separation processes by stabilizing the polyimide structure with crosslinks. In the polymerization reaction 6FDA (4,4＇-(hexafluoroisopropylidene)diphthalic anhydride) was used as dianhydride monomer and mPD (m-phenylene diamine) and DABA (diamino benzoic acid) were used as diamine monomers. With copolyimides containing strong polar carboxylic acid ups (i.e. 6 FDA-mPD/DABA 9:1) reduced plasticization was seen up to a pure CO: feed pressure of 14 atm, presumably to hydrogen bonding between the carboxylic acid groups. By chemical crosslinking of the free carboxylic acid groups of the 6FDA-mPD/DABA 9:1 with ethylene glycol, the swelling effects due to CO2 can be reduced at least up to a pure CO2 feed pressure of 35 atm, With increasing degree of crosslinking, increasing CO2/CH4 selectivity was found because of reduced swelling and polymer chain mobility. By using ethylene glycol as a crosslinking agent, CO2 permeability was not significantly lowered because the reduced chain mobility was compensated by the additional free volume caused by the crosslinks.