During the past decades there has been a great accumulation of important data on the diffusion of water molecules in polymeric solids and its effect on the mechanical and viscoelastic behavior of polymers. It has become apparent that in many cases diffusion in polymers as well as its effect exhibits features that cannot be expected from classical theories and that such departures are related to the molecular structure characteristics of polymers. In the present investigation, the mechanical and viscoelastic behavior of an epoxy resin system is studied as a function of absorbed water, temperature, and time of immersion. Water sorption was achieved by immersing the material in distilled water at constant temperature of 60 degrees C and 80 degrees C for 2, 5, 8, 13, 32, 74, 128, 266, 512, 1024, and 1536 h. Subsequently the specimens were tested in static and dynamic three-point bending tests to study their mechanical and viscoelastic behavior. The variation of T-g, tan delta, bending modulus, and Iiunction of exposure time and strength was measured as a, respective percentage of water uptake for both temperatures. Some anomalies in their behavior due to water absorption were observed, and a model for the description of the experimentally observed mechanical behavior due to hygrothermal aging is proposed. The results show that the model predictions are in good agreement with experimental findings. (c) 2005 Wiley Periodicals, Inc.