We have determined the intrinsic gas transport properties of He, H-2, O-2, N-2, CH4, and CO2 for a 6FDA-durene polyimide as a function of pressure, temperature and aging time, The permeability coefficients of O-2, N-2, CH4, and CO2 decrease slightly with increasing pressure. The pressure-dependent diffusion coefficients and solubility coefficients arc:consistent with the dual-sorption model and partial immobilization. All the gas permeabilities increase with temperature and their apparent activation energies for permeation increase with increasing gas molecular sizes in the order of CO2, O-2, N-2, and CH4. The percentages of permeability decay after 280 days of aging are 22, 32, 36, 40, 42, and 30% for He, H-2, O-2, N-2, CH4, and CO2, respectively. Interestingly, except for H-2 (kinetic diameter of 2.89 Angstrom), the percentages of permeability decay increase exactly in the order of He (kinetic diameter of 2.6 Angstrom), CO2 (3.30 Angstrom), O-2 (3.46 Angstrom), N-2 (3.64 Angstrom), and CH4 (3.80 Angstrom). The apparent activation energies of permeation for O-2, N-2, CH4, and CO2 increase with aging because of the increases in activation energies of diffusion and the decreases in solubility coefficients. The activation-energy increase for diffusion is probably due to the decrease in polymeric molar volume because of densification during aging. The reduction in solubility coefficient indicates the available sites for sorption decreasing with aging because of the reduction of microvoids and interstitial chain space.