Magnesium oxide (MgO) nanoparticles were dispersed via solution processing in poly(1-trimethylsilyl-1-propyne) (PTMSP) to form polymer nanocomposites. Transmission electron microscopy was used to determine the extent of particle aggregation in the composites. Both nanocomposite density and CO2, CH4, N-2, and H-2 permeability were influenced by nanoparticle loading. Nanocomposite densities were markedly lower than predicted by a two phase additive model. For example, in films containing 75 nominal volume percent MgO, the polymer-particle composite density was 68 percent lower than expected based on an additive model. At this loading, gas permeability coefficients were, depending on the gas, 17-50 times higher than in unfilled PTMSP at similar conditions. The changes in permeability with particle content were interpreted in terms of measured changes in gas solubility with particle content and diffusion coefficients calculated from the permeability and solubility data. (c) 2008 Elsevier Ltd. All rights reserved.