This study aims to expand structure-property relationships of diaminophenylindane (DAPI)-containing polyimides and the influence of thermal cross-linking on gas transport properties of such materials. A polyimide synthesized from hexafluoroisopropylidene diphthalic anhydride (6FDA), DAPI, and diaminobenzoic acid (DABA) in a molar ratio of 0.5/0.33/0.17, 6FDA(0.5)-DAPI(0.33)/DABA(0.17), was crosslinked by thermal decarboxylation. After cross-linking, pure gas permeability of 6FDA(0.5)-DAPI(0.33)/DABA(0.17) increased with increased crosslinking time; gas permeability increased by about 30% after cross-linking at 353 degrees C for 40 min. This increase in permeability correlated with an increase in d-spacing measured by wide angle x-ray scattering, suggesting an increase in inter-chain spacing upon cross-linking. Minimal changes in O-2/N-2 and N-2/CH4 selectivities occurred with increased thermal cross-linking time for 6FDA(0.5)-DAPI(0.33)/DABA(0.17). However, CO2/CH4 and C2H4/C2H6 pure gas selectivities increased with thermal treatment, suggesting a potential narrowing of free volume distribution.