The application of membranes in gas separation and pervaporation requires materials that are resistant to plasticizing feed streams. We demonstrate the relationship between CO2 sorption, permeability, and film swelling of a polyimide gas separation membrane and how these properties are affected by systematic changes to the polymer structure induced by thermal annealing and covalent cross-linking. Dilation of polyimide thin films (similar to120 nm) exposed to high-pressure CO2 (UP to 100 atm at 35 degreesC) was measured by in situ spectroscopic ellipsometry to decouple the effects of thermal and chemical treatments on the film swelling. The refractive index of the CO2-swollen polymer is also used to estimate the CO2 sorption for comparison against that measured on thick films (similar to50 mum) by the pressure-decay method. Differences in sorption levels in thin and thick films appear to be related to accelerated physical aging of the thin films. Both thermal annealing and covalent cross-linking of the polyimide films reduce polymer swelling to prevent large increases in the CO2 diffusion coefficient at high feed pressures. The CO2 permeability and polymer free volume strongly depend on the annealing temperature, and different effects are observed for the cross-linked and un-cross-linked membranes. The so-called "plasticization pressure" in permeation experiments (i.e., upturn in the permeation isotherm) appears to correlate with a sorbed CO2 partial molar volume of 29 +/- 2 cm(3)/mol in the polymer. Furthermore, cross-linking of high glass transition polyimides produces a much greater reduction of the CO2-induced dilation than does cross-linking of rubbery polymers such as PDMS for swelling up to 25%.