Asymmetric polysulfone/polyimide (PSF/PI) blended membranes were developed at compositions of 80/20, 50/50 and 20/80 for N-methyl-2-pyrrolidone/dichloromethane (DCM/NMP) solvent mixtures, in order to understand the mechanism of membrane formation for CO2/CH4 separation performance. Scanning electron microscopy (SEM) investigations showed that the membrane surfaces for all compositions were homogenous and their cross-sections showed non-porous top layer and porous substructure. Morphological analysis was further correlated with solubility parameter approach and coagulation value. Results showed that for NMP/DCM solvent mixtures, the solubility parameter difference to ethanol and polymeric membranes was in the order of 80/20<50/50<20/80. Among various compositions of NMP/DCM solvent mixtures, membranes prepared from (20/80) composition showed the lowest coagulation value. The compatibility of all the membrane blends was confirmed by using differential scanning calorimetry (DSC). Thermogravimetric analysis (TGA) showed that thermal stability was improved with PI addition in the membranes blends and PSF/PI-20 exhibited the maximum thermal degradation value (Td) of 549°C. While gas permeation results showed maximum CO2/CH4 selectivity values in the range of 28.70-28.22 at 2-10bar feed pressure respectively for PSF/PI-20 prepared from NMP/DCM with solvent composition of (80/20). © 2011 Elsevier B.V.