Linear polyethers possess unusually high CO2 solubility and, hence, selectivity due to the presence of accessible ether linkages that can interact with the quadrupolar moment of CO2 molecules. In this work, membranes derived from crosslinked poly(propylene glycol) diacrylate (PPGda) oligomers differing in molecular weight (M), as well as PPGda nanocomposites containing either an organically-modified montmorillonite clay or a methacrylate-terminated fumed silica are investigated and compared with highly CO2-selective poly(ethylene glycol) diacrylate (PEGda) homopolymer and nanocomposite membranes previously reported. The theological and permeation properties of PPGda depend sensitively on M, with the elastic modulus decreasing, but CO2 permeability and CO4H, selectivity increasing, with increasing M. Incorporation of either nanofiller into PPGda enhances the elastic modulus and reduces the gas permeability in the resultant nanocomposites without strongly affecting CO2/H-2 selectivity. Blending PPGda and PEGda prior to chemical crosslinking yields binary membranes that exhibit intermediate gas-transport properties accurately described by a linear rule of mixtures. (C) 2004 Elsevier Ltd. All rights reserved.