In our preceding work, the simultaneous deviation from the conventional dual-mode sorption and mobility model was observed in a case of CO2 in polystyrene (PS) membrane (glass transition temperature of pure polymer = 95 degrees C) 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 of plasticizing ability to the polymer should exist. In the present work, the sorption and permeation behavior for CO2 in NH3-plasma-treated and untreated PS membranes at 40 degrees C was investigated, where NH3-plasma treatment was executed at plasma discharge powers of 40, 80,120, and 160 W for an exposure time of 2 min. The sorption isotherm could be described by the sorption theory of Mi et al. (Macromolecules 1991, 24, 2361), where the glass transition temperature is depressed by a concentration of sorbed CO2 of plasticizing ability. NH3-plasma treatment on PS membrane had little influence on the sorption behavior of CO2 at plasma discharge powers up to 160 W. The mean permeability coefficients were somewhat increased only at a discharge power of 160 W. At CO2 pressures below 0.9 MPa, the pressure dependencies of mean permeability coefficients for CO2 in both NH3-plasma-treated and untreated PS membranes at 40 degrees C could also be simulated on the basis of the concept that only one population of sorbed gas molecules of plasticizing ability should exist. At CO2 pressures above 0.9 MPa, however, a plasticization action of sorbed CO2 had much more influence on the diffusion process rather than on the sorption one, and such a combined concept underestimated the mean permeability coefficient. 2006 Wiley Periodicals, Inc.