One of the problems which lie in separating CO 2 from CH 4 or N 2 by glassy polymeric membranes is the plasticization that occurred at higher partial pressures than specific pressure. This phenomenon causes the reduction in gas separation ability of glassy membranes. It is known that the permeability of mixed gases is different from pure gases due to competitive sorption. In the first part of this investigation, a model for predicting the permeability and the effective diffusion coefficient of mixed gases in the presence of plasticization were proposed. In this model, it was assumed that the diffusion coefficient for all components was exclusively function of plasticizing component. Also, the partial immobilization model was employed to determine the fraction of mobile sorbed gases. Then, three groups of the experimental data from the literature including the permeation of CO 2 /N 2 and CO 2 /CH 4 through asymmetric PI/PES and CA membranes, and also the permeation of CO 2 /CH 4 through dense CA membrane, compared to the model and the coefficients of the model were determined. Plasticization parameter decreased for both components by increasing the fraction of second component in the feed. It means that plasticization of glassy polymers was suppressed. This decrease was caused by competitive sorption between CO 2 and the second component. Also, Immobilization factor of all components decreased with the increase in the fraction of second component. The effective diffusion coefficient for pure CO 2 was significantly pressure-dependent due to the higher degree of the plasticization. However, with increasing the fraction of second component in the feed, this dependency decreased. In the second part, the effect of thermal treatment and also adding TiO 2 nanoparticles on the performance of asymmetric PEI membrane in separating CO 2 from N 2 were investigated. Thermal treatment at temperatures lower and higher than T g were done. The results show that for the untreated membrane, plasticization occurred with the increase in upstream pressure. For the membranes which treated at 120, 160 and 200 °C for 5 hr, plasticization pressured increased to 6 bars. For the membrane which treated at 220 °C for 10 min, plasticization was not observed to 10 bars. Moreover, permeances of the gases through the treated membranes were lower than untreated membrane due to densification and the reduction in free volumes in the selective layer. For PEI/TiO 2 membranes, with adding 1 and 2 wt% of TiO 2 , plasticization pressure increased to 4 and 5 bars due to the reduction in the movement of polymer chains and the increase in the resistance of the membrane. For PEI/5%TiO 2 , plasticization occurred due to agglomeration of nanoparticles. Moreover, permeation of CO 2 and N 2 through PEI/TiO 2 membranes was higher than pure PEI membrane. In addition, for PEI/TiO 2 membranes with 1 and 2 % of TiO 2 , CO 2 /N 2 ideal selectivity were increased whereas for the membrane with 5% TiO 2 , the selectivity decreased due to agglomeration of nanoparticles and the presence of defect in the selective layer of the membrane. Also, prepared membranes were characterized by SEM,،FTIR, XRD, TG and DMT analyses.