Composite membranes, prepared by coating PPO on top of a 12kDa MWCO ultrafittration membrane (Osmonics-HO51), showed enhanced gas permeability ratio and separation factor for CO2/CH4 gas system, as well as some decrease in the permeability Of CO2 gas, in comparison to the dense homogeneous PPO membrane. Average permeability ratio, CO2/CH4, being reported in this paper is 37 for composite membranes, and 17 for homogeneous membranes. The CO2 permeability obtained for composite and homogeneous PPO membranes were 80 and 92 Barrer, respectively. AFM observations showed that the coated layer of the composite membrane possessed graded compactness across the depth, rough and large polymer aggregates on top surface while smooth and fine polymer aggregates on bottom surface. This morphology profile was opposite to that observed for the dense homogeneous membrane. Based on AFM observations and solvent evaporation kinetics, it is postulated that the enhancement in the selectivity and the drop in the gas permeability of the coated layer, in comparison to that of the dense homogeneous membrane, is due to the densification and compaction of the polymer adjacent to the support membrane interface. This is the direct result of the migration of solvent toward the support membrane and the partial draw of the solvent into the support membrane pores. This has produced a smoother bottom surface as a result of (i) the formation of small nodules, and (ii) the fusion of these small nodules by the compressive forces that have developed due to the shrinkage of the swollen support substrate. These compressive forces have caused the bottom surface to wrinkle. (c) 2005 Published by Elsevier B.V.