Polyimide (PI) based membranes are commercially attractive for CO2/CH4 separations due to their high selectivity and permeance. However, PI membranes prepared from commercially available Matrimid(R) become unsuitable for industrial applications at high CO2 partial pressure due to plasticization. Blending appropriate polymers with Matrimid(R) is known to increase this critical plasticization pressure. However, most open literature studies are based on thick symmetric films. Moreover, stability of the membranes as a function of temperature and composition of binary gas mixtures has not systematically been studied yet. The study so far of single gas feeds with thick dense membranes strongly limits the relevance with respect to the actual industrial scale separations. In this work, asymmetric membranes were prepared from Matrimid(R), Ultrason polysulphone (PSf) and blends thereof with different ratios. Physical properties (T-g, FFV, density and d-spacing) of the membranes were determined in addition to SEM-based membrane cross-section morphologies to better understand the membrane performance. The improvement of the Matrimid(R) membranes by blending was studied under elevated temperature and at varying CO2 feed composition. A membrane prepared from a 3:1 (Matrimid(R):PSf) blend ratio showed consistent increase in selectivity at high CO2 feed composition (up to 90 vol%), elevated temperature (up to 95 degrees C) and pressure (up to 14 bar). On the contrary, unblended Matrimid(R) membrane performance deteriorated under operating conditions above 90 vol% CO2 feed composition, 65 degrees C and 12 bar. Moreover, prepared asymmetric membranes showed much higher permeances than corresponding dense membranes without too much compromising selectivity. (C) 2010 Elsevier B.V. All rights reserved.