We have determined the gas transport properties of He, H 2 , O 2 , N 2 , and CO 2 for 6FDA-durene homopolymer and 6FDA-durene/pPDA copolyimides. The 6FDAdurene exhibits the highest permeability with the lowest selectivity. Permeability of copolymers decreases with increasing 6FDA-pPDA content, while permselectivity increases with an increase in 6FDA-pPDA content. 6FDA-durene/pPDA (50/50) and 6FDA-durene/pPDA (20/80) materials have O 2 and CO 2 permeabilities greater than those calculated from the addition rule of the semilogarithmic equation. These higher deviations from the additional rule of the semilogarithmic equation are mainly attributed to the fact that these copolyimides have higher solubility coefficients than those calculated from the additive rule. The T g s of 6FDA-durene/pPDA copolyimides decrease with an increase in 6FDA-pPDA content. T g s predicted from the Fox equation are lower than the experimental data, and the their difference increases with an increase in pPDA content, implying the copolyimides of 6FDA-durene/pPDA may have greater interstitial space among chains because of the conformation difference, and thus create more fraction free volume compared with the ideal case of simple volume addition. Density measurements also suggest these two copolymers have greater free volumes and the fractions of free volume, which supporting the gas transport results. The thermal stability and ␤-relaxation temperature have also been studied for these copolymers.