Water absorption at 23 and 100-degrees-C has been studied in three materials based on a single type of urethane-methacrylate resin : the neat resin, a rubber-toughened blend containing 15 wt % of core-shell modifier, and a filled rubber-toughened resin containing 46 wt % silica plus 8 wt % core-shell modifier. In all six absorption experiments, water diffusion followed Fick’s law. However, in neat resin at 100-degrees-C, the period of Fickian diffusion, during which the material appeared to reach saturation, was followed by an additional period of water uptake, which was accompanied by formation of internal disc cracks up to 1 mm in diameter. At both 23 and 100-degrees-C, fracture of water-saturated neat resin was defect dominated : cracks were initiated at inorganic impurities between 40 and 80 mum in diameter. The adverse effects of water are attributed to hydration and consequent swelling of these insoluble impurities. No internal cracking was detected in either the rubber-modified or hybrid resin, which have higher a K(IC)s than the neat resin.