Objectives. The purpose of this study was to assess the effects of aging experimental dimethacrylate resin composites in water at 37°C for periods up to 6 wk by measuring the variations in fracture toughness (Kc), elastic modulus (E), fracture energy (Gc), and water sorption. Methods. Six experimental resins were formulated from dimethacrylate resins, and were filled to 86 wt% (ca. 70 vol%) with treated inorganic filler to form six experimental composites. The fracture toughness was determined using a double torsion technique, the elastic modulus was measured in flexure, and the fracture energy was calculated from the fracture toughness and elastic modulus. Results. As a result of aging in water, Kc and the Gc increased, and the elastic modulus decreased, but all values approached a plateau near 6 wk. Water sorption also occurred during this period, mainly during the first 2 wk. Significance. Variations in the mechanical properties are interpreted as being due to plasticization of the resin matrix by water, which appears to lower the yield stress and increase in the size of the plastic zone ahead of the crack, thereby causing the observed increase in Gc and Kc. After approximately 6 wk, no further changes in properties occurred.