Polyetherimide (PEI) was used as a polymeric additive for preparing an asymmetric polyethersulfone (PES) membrane for the separation of CO2 from CH4. In pure gas experiments, the higher skin layer thickness and the lower porosity of the sub layer for the membrane prepared from the polymer blend with the composition of 98:2 lead to an increase in CO2/CH4 selectivity and a decrease in the CO2 permeance in contrast with a pristine PES. For higher PEI contents, the higher fractional free volume of the membranes improves the gas permeance and reduces the CO2/CH4 selectivity. The incorporation of PEI in PES reduces the CO2 sorption in PES via decreasing the non-equilibrium free volume and imparts antiplasticization properties to the membrane. (C) 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.