In this work, Polydimethyl siloxane/polysulfone (PDMS/PSF) membrane was developed by dip coating of developed PSF membrane. Developed membranes were characterized in terms of thermal analysis, glass transition temperature, and membrane morphology. Performances of the developed membranes were tested using gas permeation equipment within the pressure range of 2-10 bar. To study the water swelling in membrane, developed membranes were soaked in distilled water for 5 minutes. Gas permeation in membrane was investigated before and after swelling. Based on the results, increase in CO2 permeance was observed due to plasticization in membrane before swelling. However, CO2 permeance decreased after swelling in membrane. Swelling affected the separation performance of membrane by decreasing its permeance. While, CO2/CH4 selectivity has increased after membrane swelled. Hence, swelling resulted in membrane contraction, which contributed to the increase of selectivity. Pure polysulfone membrane exhibited higher degree of swelling, whereas PDMS/PSF membranes demonstrated substantial high resistance toward swelling. Based on swelling degree of PDMS/PSF composite membrane, this membrane can be considered as a potential water (swelling) resistant polymeric membrane for CO2/CH4 separation. © 2016 The Authors.