Following our preceding review article in 2000 [1], we principally discussed the sorption and diffusion behavior for CO₂ in glassy polymer membranes near the glass transition temperature. 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 CO₂ molecules of plasticizing ability to the polymer should exist; a gas-polymer-matrix model proposed by Raucher and Sefcik [5] combined with the sorption theory of Mi et al. [3]. Thereat, the glass transition temperature is depressed by a concentration of sorbed CO₂ of plasticizing ability. The simultaneous deviation from the conventional dual-mode sorption and mobility model was observed in cases of CO₂ in poly(4-methyl-1-pentene) membrane (PMP, glass transition temperature of pure polymer T_g = 34^oC) at 20^oC, in polystyrene membrane (PS, T_g = 95^oC) at 60^oC and 70^oC and in cellulose triacetate membrane (CTA, T_g = 80^oC) at 50^oC and 60^oC. The sorption and permeation behavior influenced by the plasticization action of sorbed CO₂ molecules for these systems, could be simulated well by the above combined model. Next, the sorption and permeation behavior for CO₂ in NH₃-plasma-treated and untreated PS membranes at 40^oC was analyzed by the proposed combined model. The pressure dependencies of mean permeability coefficients for CO₂ in both NH₃-plasma-treated and untreated PS membranes at 40^oC, being considerably lower than the glass transition temperature of pure polymer (95^oC), for CO₂ pressures below 0.9 MPa, could also be simulated in terms of the proposed combined model. Above 0.9 MPa, a plasticization action of sorbed CO₂  had an influence on the diffusion process rather than on the sorption one, like in the system of   CO₂ - NH₃-plasma-treated and untreated poly (methyl methacrylate) (PMMA, T_g = 105^oC) membranes at 30^oC and CO₂ - NH₃-plasma-treated and untreated CTA membranes at 30^oC. The pressure dependencies of mean permeability coefficients for the latter system (CO₂ in both NH₃-plasma-treated and untreated CTA membranes at 30^oC) can qualitatively be described by the proposed combined model.