Plasticization phenomena can significantly reduce the performance of polymeric membranes in high-pressure applications. Polyetherimides (PEIs) are a promising group of membrane materials that combine relatively high CO2/CH4 selectivities with high chemical and thermal stability. In this work sorption, swelling, and mixed gas separation performance of 3,3',4,4'-oxydiphthalic dianhydride (ODPA)-based PEI polymers, with 1, 2 or 3 para-aryloxy substitutions in the diamine moeiety, is investigated under conditions where commercial membranes suffer from plasticization. Particular focus is on the influence of the amount of para-aryloxy substitutions and the film thickness. Results are compared with those of commercially available polymeric membrane materials (sulphonated PEEK, a segmented block-co-polymer PEBAX and the polyimide Matrimid). The glassy polymers display increasing CO2 sorption with increasing T-g. The larger extent of sorption results from a larger non-equilibrium excess free volume. Swelling of the polymers is induced by sorption of CO2 molecules in the non-equilibrium free volume as well as from molecules dissolved in the matrix. Dilation of the polymer is similar for each molecule sorbed. Correspondingly, the partial molar volume of CO2 is similar for molecules present in both regions. Mixed gas separation experiments with a 50/50% CO2/CH4 feed gas mixture showed high CO2/CH4 selectivities for the ODPA PEI films at elevated pressure. This shows that these materials could potentially be interesting for high-pressure gas separation applications, although additional gas permeation experiments using different feed gas compositions and thin films are required. (C) 2010 Elsevier Ltd. All rights reserved.