The water-transport, mechanical, and chemical-structure changes in various vinyl ester, novolac, and urethane-modified vinyl ester thermosets exposed to water at 50 to 95 degrees C for times up to 1000 days have been Studied within the framework of a larger study of osmotic blistering in fiber reinforced plastics (FRP) process components. The water sorption saturation concentration did not reach a steady-state value but gradually increased in many cases upon long-term exposure. The diffusion coefficient was not significantly affected. Infrared spectroscopy and gas chromatography-mass spectrometry indicated that the net mass loss from the thermosets on immersion in water was due to the leaching of non-reacted styrene, monomer, and additives. It is suggested that this, together with polymer relaxation processes (as measured on specimens under tension in water at 80 degrees C), is the primary reason for the time-dependent increase in the water saturation concentration. Infrared spectroscopy indicated that, even at the highest temperatures, hydrolysis of the polymer ester groups was small. Correlations were found between the styrene content in the uncured thermosets, the estimated solubility parameters, and the sorption and diffusion coefficients. (C) 2009 Wiley Periodicals, Inc. I Appl Polym Sci 116:1057-1067, 2010