The gas transport properties of He, H-2, O-2, N-2, and CO2 for 6FDA-durene, 6FDA-mPDA, and their copolyimides have been investigated as a function of the composition, pressure, and temperature. The permeabilities, diffusion coefficients, and solubility coefficients follow the simple additional rules and Arrihenuis relationships for 6FDA-durene/mPDA polyimides. The permeability decreases with an increase in pressure except He, and in the kinetic diameters of the penetrant molecules with the order of He, CO2, O-2, and N-2. The diffusion coefficients of O-2, N-2, and CO2 increase with increasing pressure and temperature. Interestingly, for these polyimides, the diffusion coefficient of O-2 is larger than that of CO2. This may be caused by the strong quadrupole moment of the CO2 molecules or the difficulty in estimating the accurate diameter of CO2 molecules. The solubility coefficients decrease in the order of inherent condensability of the penetrant gases, namely CO2, O-2, and N-2. In addition, WAXD results show that 6FDA-durene/mPDA polyimides are amorphous. The presence of a single glass transition temperature for these copolyimides indicates that these polyimides can be assumed to be random copolymers. They have excellent thermal stability exhibiting degradation temperature in a range of 493 to 548 degreesC.