To suppress undesirable plasticization effects of condensable gas (CO2) in high pressure gas-separation process, a surface-modified poly(methyl methacrylate) (PMMA) membrane prepared by a plasma treatment was synthesized in this study. The pressure dependencies of permeability were analyzed to characterize the plasticization phenomenon and how it can be controlled by the plasma modification. FTIR-ATR and TGA results suggest that the plasma treatment leads to surface crosslinking of PMMA membrane. Thus, the plasticization effect resulting from sorbed CO2 on the Ar-plasma-treated PMMA membrane was significantly reduced, but the reduction in permeability was lower than that of other crosslinking modification methods. The reduction of the CO2 permeability contributes to a major part to the selectivity of He/CO2 for the Ar-plasma-treated PMMA membranes. (C) 2004 Wiley Periodicals, Inc.