The gas-transport properties of poly[2,6-toluene-2,2-bis(3,4-dicarboxylphenyl)hexafluoropropane diimide] (6FDA-2,6-DAT) have been investigated. The sorption behavior of dense 6FDA-2,6-DAT membranes is well described by the dual-mode sorption model and has certain relationships with the critical temperatures of the penetrants. The solubility coefficient decreases with an increase in either the pressure or temperature. The temperature dependence of the diffusivity coefficient increases with an increase in the penetrant size, as the order of the activation energy for the diffusion jump is CH4 > N-2 > O-2 > CO2. Also, the average diffusion coefficient increases with increasing pressure for all the gases tested. As a combined contribution from sorption and diffusion, permeability decreases with increases in the pressure and the kinetic diameter of the penetrant molecules. Even up to 32.7 atm, no plasticization phenomenon can be observed on flat dense 6FDA-2,6-DAT membranes from their permeability-pressure curves. However, just as for other gases, the absolute value of the heat of sorption of CO2 decreases with increasing pressure at a low-pressure range, but the trend changes when the feed pressure is greater than 10 atm. This implies that CO2-induced plasticization may occur and reduce the positive enthalpy required to create a site into which a penetrant can be sorbed. Therefore, a better diagnosis of the inherent threshold pressure for the plasticization of a glassy polymer membrane may involve examining the absolute value of the heat of sorption as a function of pressure and identifying the turning point at which the gradient of the absolute value of the heat of sorption against pressure turns from a negative value to a positive one. (C) 2003 Wiley Periodicals, Inc.