Network copolymers, prepared by photopolymerizing poly(ethylene glycol) diacrylate (PEGDA: CH2=CHCOO(CH2CH2O)(14)OCCH=CH2) and poly(ethylene glycol) methyl ether acrylate (PEGMEA: CH2=CHCO(OCH2CH2)(8)OCH3), have been studied for mixed-gas CO2/H-2 and CO2/CH4 separations. Gas separation properties in the networks are sensitive to copolymer composition, temperature, and carbon dioxide fugacity in the feed. In conventional approaches, the effect of temperature can be described by the Arrhenius equation, while the influence of copolymer composition and fugacity can only be described empirically; the corresponding models require a large number of adjustable parameters. However, as shown here, the three factors influencing gas transport correlate well with free volume, and a simplified free volume model with only two adjustable parameters can satisfactorily describe the effects of copolymer composition, temperature, and CO2 fugacity on pure- and mixed-gas transport properties. Copolymer composition and temperature can be directly related to the fractional free volume, while CO2 fugacity of the feed can be correlated with free volume via the glass transition temperature of the polymer-gas mixture, which is computed using Chow's model. (c) 2007 Elsevier B.V. All rights reserved.