The changes in permeability of N-2, O-2, He, CH4 and CO2 of thin films (similar to 300 nm) of crosslinked and uncrosslinked 6FDA-based polyimides with DABA units in the chain were monitored as a function of aging time at 35 degrees C. The crosslinked polyimides have significantly lower permeability than the corresponding uncrosslinked material. Over about 2000 h of aging, the oxygen permeability decreased by two- to three-fold depending on the polymer structure. The changes in density for the crosslinked and uncrosslinked polyimides with aging time were calculated from refractive index measurements made by ellipsometry using the Lorentz-Lorenz equation. An excellent correlation between the volumetric aging rate and the permeability reduction rate is observed for these polyimides and other glassy polymers. On a relative basis, it seems that the crosslinked polyimide films show slightly higher or about the same aging rates than uncrosslinked polyimide films. Periodic exposure to CO2 has a significant effect on the permeability and selectivity and how they change with physical aging for the uncrosslinked polyimides but the effects for crosslinked films are much less. Thus, crosslinking appears to be a viable strategy for mitigating the plasticization or conditioning effects of CO2 exposure. (c) 2006 Elsevier B.V. All rights reserved.