In this study, the effect of the molecular weight of polyehtyleneglycol (PEG) on the gas permeability and selectivity of a series of polyvinylchloride/polyehtyleneglycol (PVC/PEG) blend membranes was investigated. The molecular weight of polyethylne glycol ranged from 400 to 4000. PVC/PEG blend membranes were prepared via thermal phase inversion of PVC/PEG/dimethylformamide solution at 60 degrees C. The permeabilities of pure CO2, C-4, O-2, and N-2 gases through prepared membranes were determined at room temperature (25 degrees C) and 20 bar feed pressure. The blend membranes having PEG with 4000 molecular weight showed higher values of CO2 permeability, and CO2/CH4 and CO2/N-2 ideal selectivities (the ratio of pair gas permeabilities) compared with other blend membranes containing PEGs of lower molecular weights. It was also found that the gas permeabilities of PVC/PEG4000 blend membrane rapidly increased with an increse in PEG concentration (weight fraction). The PVC/PEG blends containing 10, 20, and 30 wt % of PEG4000 showed CO2 permeabilities equal to 0.84, 2.38, and 5.82 Barrer (1 Barrer = 10-10 cm3 (STP) cm/cm(2) s cmHg), respectively. The CO2/N-2 ideal selectivities of 10, 20, 30 wt % of PEg/4000PVC blend membranes were 49, 70, and 109, respectively. This is a great improvement compared with the pure PVC membrane, which showed CO2 permeability and CO2/N-2 ideal selectivity of 0.13 Barrer and 14.5, respectively. These results and comparison of CH4 and N-2 permeabilities of PVC membrane and PVC/PEG blend membranes indicated that the gas solubility is dominant in the permeation of gases through abovementioned blend membranes. (c) 2008 Wiley Periodicals, Inc.