The performance of zeolite and low molecular weight additive incorporated polyethersulfone (PES) membranes on the separation of CO2/CH4 mixtures at 35 degrees C was investigated. Four types of membranes, pure PES, PES/ 2-hydroxy 5-methyl aniline (HMA), PES/SAPO-34 and PES/SAPO-34/HMA, were prepared by solvent evaporation method. The CO2 concentration in the feed was varied between 5 and 70% by volume. PES/SAPO-34 membranes had a total permeability coefficient of 3 Barrer for an equimolar mixture, which was the highest permeability among all membranes. The highest separation factor, which was approximately 40 for CO2 over CH4, was obtained from PES/SAPO-34/HMA(4%) membrane. The addition of HMA considerably influenced the separation performance of PES/SAPO-34 membrane. The permeabilities increased with increasing concentration of CO2 in the feed whereas the separation factors of membranes were independent of feed composition, which introduces a major advantage from industrial point of view since the membranes can be used with the same performance in the separation of CO2/CH4 mixtures regardless of mixture composition. (C) 2012 Elsevier B.V. All rights reserved.