Understanding the mechanism of asymmetric membrane formation is very important in order to produce membranes of desirable morphology for gas separation applications. The mechanism of asymmetric PC membrane formation for varying solvent-non-solvent pairs in casting solutions and their relation with CO2/CH4 separation performance have been investigated in this work. The solvents used were dichloromethane (DCM) and chloroform while ethanol (EtOH), propanol (PrOH) and butanol (BuOH) were selected as non-solvents. The membrane morphologies are explained on the basis of solvent and non-solvent evaporation, solubility parameter and coagulation value. Experimental results showed that delayed demixing of casting solution produces less porous substructure. Scanning electron microscopy (SEM) was used to observe the membrane morphologies. The membrane performance was evaluated by permeation of CO2 and CH4 at different feed pressures. Propanol- and butanol-based membranes showed promising performance. High CO2/CH4 ideal selectivities were obtained in the pressure range of 1-5 bar (alpha CO2/CH4 = 93-18 for DCM-PrOH membrane; alpha CO2/CH4 = 70.39-6.85 for DCM-BuOH membrane; alpha CO2/CH4 = 173.88-2.86 for chloroform-PrOH membrane; alpha CO2/CH4 = 112.09-19.99 for chloroform-BuOH membrane). These results showed that by varying the solvent-non-solvents pair of casting solution could control the morphology and eventually affect the performance of asymmetric PC membranes. (C) 2008 Elsevier B.V. All rights reserved.