The deviation from the conventional dual-mode sorption and mobility model for a gas in glassy polymer membranes has separately been studied thus far, and to simulate sorption and diffusion behavior, an extended dual-mode sorption model and a modified dual-mode mobility model, respectively, have been proposed independently. However, simultaneous deviation from the conventional dual-mode sorption and mobility model was observed in cases of CO2 in poly-4-methyl-1-pentene membrane at 20 degrees C and in polystyrene membrane at 60 and 70 degrees C. The plasticization effect of sorbed CO2 on both the sorption and diffusion processes tends to be brought about in glassy polymer membranes near the glass transition temperature. The behavior was simulated based on the concept that only one population of sorbed gas molecules exists.