Preparation of pure polysulfone (PSf) membrane for CO2/CH4 separation was aimed in this study. Accordingly, the effects of different variables such as: type and concentration of alcohol as external nonsolvent in the coagulation bath, solvent type in the casting solution and also presence of butanol (BuOH) as internal nonsolvent in polymer solution were examined. CO2 and CH4 permeabilities of prepared membranes in different coagulation baths follow this order: ethanol-50% (EtOH-50%) > isopropyl alcohol-50% (IPA-50%) > ethanol-100% (EtOH-100%) > IPA-100%. According to scanning electron microscopy photographs, membrane asymmetry decreased in higher concentration of alcohols and a high symmetric membrane was prepared using IPA-100% as external nonsolvent. CO2/CH4 selectivity improved in the following order: IPA-100% > EtOH-100% > IPA-50% > EtOH-50%. Then, a high CO2/CH4 selectivity (36.40) was obtained employing pure IPA in coagulation bath. When a mixture of NMP/THF was used instead of NMP as solvent, CO2/CH4 selectivity increased from 7.10 to 18.50. Thickness of membranes decreased from 124.70 to 72.11 mu m by addition of BuOH concentration from 0 to 10 wt% as internal nonsolvent. Consequently, an enhancement in gas permeability was observed in higher BuOH concentrations. (C) 2013 Society of Plastics Engineers