Dental restorative composite resins of the monomer bisphenol A glycerolate dimethacrylate (BisGMA/TEGDMA; 50/50, mass ratio), with different initial water contents in monomer mixtures and subject to various postcuring conditions, were prepared. The effects of water contents and postcuring conditions on photopolymerization rate, degree of methacrylate double bond conversion, and mechanical properties (flexural strength and elastic modulus) were systematically investigated. Within the scope of this investigation, the initial water content was not found to significantly affect the mechanical properties of the photocured composite resins. Nevertheless, the addition of distilled water into the monomer mixture slightly increased the degree of methacrylate double bond conversion, and slightly slowed down the photocuring process, especially when the amount of water was oversaturated. Postcuring conditions of 37degreesC vapor or water aging increased the degree of methacrylate double bond conversion, but made the mechanical properties of the composite resins less desirable, especially under water-aging conditions. Postcuring treatment of the composite resins in a 100degreesC vacuum (similar to 27.9 KPa) oven for 1 day improved both the flexural strength and the elastic modulus by at least 30%, but the improvements quickly diminished upon subsequent 37degreesC water aging. (C) 2004 Wiley Periodicals, Inc.