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This study reports 6FDA:BPDA-DAM polyimidederived hollowf iber carbon molecular-sieve (CMS) membranes for hydrogen and ethylene separation. Since H 2 /C 2 H 4 selectivity is the lowest among H 2 /(C 1 -C 3 )h ydrocarbons,a n optimizedC MS fiber for this gas pair is useful for removing hydrogen from all-cracked gas mixtures.Aprocess we term hyperaging provides highly selective CMS fiber membranes by tuning CMS ultramicropores to favor H 2 over larger molecules to give aH 2 /C 2 H 4 selectivity of over 250. Hyperaging conditions and ah yperaging mechanism are discussed in terms of an expedited physical aging process,w hich is largely controlled by the hyperaging temperature.F or the specific CMS material considered here,ahyperaging temperature beyond 90 8 8Cbut less than 250 8 8Cworks best. Hyperaging also stabilizes CMS materials against physical aging and stabilizes the performance of H 2 separation over extended periods.This work opens ad oor in the development of CMS materials for the separation of small molecules from large molecules.