In this work, the transport properties of water in various thermoset resins typically used for fiber-reinforced plastic process equipment (vinyl ester, novolac, bisphenol-A-epoxy-based polyester, and urethane-modified vinyl ester) were systematically studied with a focus on the effects of water activity. Sorption and desorption studies were carried out in aqueous solutions (water activity = 0.781) with different salts (NaCl, MgCl2, Na2SO4, and KCl) and one nonionic substance (sucrose) within the temperature interval 65-95 degrees C. It was found that the water concentration in thermoset resins with different chemical structures could be predicted from the water activity, regardless of the actual solute (salt or sucrose) in the aqueous solution. The water quasi-equilibrium concentration decreased with decreasing water activity, whereas the average diffusion coefficient was considered to be independent of water activity in the studied water activity range. A relationship, based on a power law fit, was established describing the water concentration as a function of the water activity and the concentration of pure water. The relationship was independent of resin chemistry and temperature. POLYM. ENG. SCI., 52:717-724, 2012. (C) 2012 Society of Plastics Engineers