An equation based on the segmental/chain mobility in free volume theory was derived to correlate the gas permeation coefficient and the aging time. An accelerated physical aging of a fluoropolyimide was observed and employed to validate this equation. A strong thickness-dependent aging process was found by employing pure O-2 and N-2 tests to monitor the change of gas permeation properties as a function of aging time. Experimental results also suggest that chain rigidity and configuration play important roles in physical aging. As a result, the thin 2,2'-bis(3,4'-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) based polyimide films studied here have different permeability versus time relationship from conventional polyarylate in the early stage of aging, and the experimental data seem to fit the proposed equation well. (C) 2004 Wiley Periodicals, Inc.