Asymmetric membranes of semi-interpenetrating polymer networks (semi-IPN) were prepared with commercial poly (ether imide) (ULTEM (R)) and poly (ethylene glycol) diacrylate (PEGDa) in 1-methyl-2-pyrrolidinone (NMP). The selectivity and permeance of pure and mixed gases using carbon dioxide (CO2) feed concentrations of 10-40% in nitrogen (N-2) or methane (CH4) were measured by the constant pressure and variable volume method at an absolute feed pressure of 1.35 MPa and 22 degrees C. The pure gas selectivity matched the mixed gas selectivity values at different feed concentrations, which indicated absence of plasticization. The fugacity based CO2/N-2 selectivity of a semi-IPN with 6% PEGDa solids content reached 50 +/- 4, which is comparable to the pure gas selectivity of a dense PEGDa film (alpha = 54) and is significantly higher than the dense film selectivity of PEI (alpha = 28) The selectivity for CO2/CH4 mixtures is 43 +/- 10. comparable to the dense film properties of PEI (alpha = 39) and not the dense film selectivity of PEGDa (alpha = 20). The PEGDa/PEI semi-IPN membranes displayed synergistic properties, where the selectivity approached the higher value of the two materials used in making the semi-IPNs (C) 2010 Published by Elsevier B V