Bis-GMA (2,2-bis-[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane) is a viscous hygroscopic monomer which is used with triethyleneglycol-dimethacrylate (TEGDMA) for dental restorations. Bis-GMA was silylated with dimethyl-isopropyl-siloxane and further polymerized in order to increase water resistance and viscosity. The viscosity of the silylated monomer, Sil center dot Bis-GMA, was 50 times lower than that of the parent monomer. After 1 month in water, poly(Bis-GMA/TEGDMA) absorbed 2.6% water and the silylated polymer, poly(Sil center dot Bis-GMA), only 0.56%. During this process water extracted residual monomer from each polymer. The behavior of water sorption and desorption as a function of time in poly(Sil center dot Bis-GMA) was completely different from that shown by poly(BisGMA/TEGDMA). The difference is discussed in terms of diffusion coefficients. Initially, water advancing contact angles (theta(ADV)) were 75 degrees and 95 degrees, respectively. After I month in water both polymers showed a reduction of about 20 degrees in theta(ADV). In poly(Bis-GMA/TEGDMA), the reduction in theta(ADV) obey to water absorption and bulk plasticization; it showed a reduction of 15 degrees C in glass transition temperature, T-g. In contrast, the reduction in theta(ADV) in poly (Sil center dot Bis-GMA) obeyed to water adsorption and reorientation of the molecules at the surface in contact with the water phase; it only showed a change of 2 degrees C in T-g. Contact angle hysteresis provided further evidence about plasticization. According to our results poly(Sil center dot Bis-GMA) is more stable in water than poly(Bis-GMA/TEGDMA). (C) 2007 Wiley Periodicals, Inc.