Although propylene/propane separation remains a challenge for industrial processes, carbon molecular sieve membranes (CMSMs) have the potential to replace traditional separation methods. A high molecular weight crosslinkable polyimide was utilized to fabricate CMSMs, which showed pure gas permeabilities in excess of 400 barrers with propylene/propane selectivities as high as 25. Mixed gas (C3H8:C3H6 50:50) measurements yielded a propylene permeability of 257 barrers and a selectivity of 20. CMSMs from thermally precrosslinked polymer precursors demonstrated a 98% propylene permeability retention after aging for 20 days under vacuum. Active gas flow conditions resulted in slightly lower permeability retention (92.5%) after 15 days of testing.