Methyl methacrylate (MMA) was copolymerized with vinyltriethoxysilane (VTES) using Co-60-gamma-radiation at varying volume fractions of the liquid monomers in the feed. Thin copolymer films were prepared by casting tetrahydrofuran solutions of the copolymers on a mercury surface. The effect of the mole fraction of VTES in the copolymer on the glass transition temperatures, mechanical properties and water vapor permeabilities were investigated. Glass transition temperatures and mechanical properties decreased with increasing VTES content in the copolymer. However, the water vapor permeabilities were found to increase with the VTES content. The increased water vapor permeation was attributed to increased water solubility in the copolymers due to the presence of Si-O-bonds.