The miscibility and gas separation performance of Matrimid and sulfonated aromatic poly(ether ether ketone) (S-PEEK) blend membranes prepared by solution casting method were studied. A S-PEEK polymer with fixed degree of sulfonation was used for membrane synthesis. Both Matrimid and S-PEEK were physically miscible over the whole composition range, as confirmed by visual observation and DSC studies. The effect of variation in composition on gas permeability and selectivity of blend membranes was investigated. The gas permeability and selectivity values fall in between those of the individual polymers, varying systematically with variation of S-PEEK content in the blend. The effect of feed pressure on CO2 permeation was linked to CO2 plasticization of the membranes. The Matrimid component of the blend membrane was cross-linked with p-xylenediamine to improve the anti-plasticization properties of the blend. In order to further study the stability and potential industrial application of these membranes, they were tested at different conditions of feed pressure, temperature and CO2 feed concentration. (C) 2011 Elsevier B.V. All rights reserved.