In this study, polydimethylsiloxane (PDMS)-coated polyethersulfone (PES) composite membrane was prepared for gas separation. 'Film casting' and 'dip-coating' techniques were used for producing selective PDMS layer on the surface of the PES support. The effects of coating technique and conditions including coating solution concentration and curing temperature on permselectivity of CO2, CH4, and N-2 were investigated. The prepared PES support did not provide any selectivity to the gases. When the concentration of PDMS coating solution was increased, initially permeability of CO2 was rapidly dropped and then gradually reached to an almost constant value. The optimum concentration of coating solution was 5 wt%. Curing temperature showed no pronounced effect on the CO2 permeability and selectivity. In 'film casting' method, double coating showed superior permeability and selectivity. However, triple 'dip-coating' was promising. The selectivity of composite membrane prepared by 'dip-coating' was higher than 'film casting' method. CO2/N-2 and CO2/CH4 selectivity of five sequential dip-coated composite membranes was 45.5 and 9.3, respectively. POLYM. ENG. SCI., 2013. (c) 2013 Society of Plastics Engineers