This paper reports pure and mixed gas CO2/H-2 and CO2/CH4 membrane separation performance of a highly permeable poly(ethylene oxide) based multi-block copolymer. Permeation and sorption properties have been studied over a wide temperature (-10 degrees C to +35 degrees C) and pressure range (up to 25 bar partial pressure of CO2). In particular, we address the effect of plasticization by CO2. A strong dependency of CO2 permeability on CO2 concentration in the polymer matrix was observed in pure and mixed gas experiments. Plasticization effects increased the permeability of H-2 and CH4 in mixed gas experiments compared to their pure gas values. The H-2 permeability was less influenced by plasticization than the CH4 permeability due to H-2's smaller kinetic diameter. As a result, mixed gas selectivities were systematically lower than pure gas selectivities. This difference between mixed and pure gas selectivity is exclusively dependent on the CO2 concentration in the polymer matrix, which can change with temperature or CO2 fugacity. Remarkably, the difference between ideal selectivity and mixed gas selectivity scales linearly with the CO2 concentration in the polymer for all pressures and temperatures considered. (C) 2010 Elsevier B.V. All rights reserved.