Absorbed water can generally degrade most polymer materials with many desired thermal and mechanical properties. In this work, a series of two-step molecular dynamics simulations have been carried out to study the moisture effects on an epoxy-amine polymer network. Copolymer of diglycidyl ether bisphenol A (DGEBA) and isophorone diamine (IPD) was investigated together with its four moist networks with concentrations of water 1.30, 2.54, 5.02 and 11.5 wt%. Simulated results clearly indicated that with addition of water molecules the density of the polymer system decreases whereas the mobility of the network chains increases at higher water concentration. However, at lower water concentration the results present the contrast trends. These differences resulting from water presence can be attributed to the interplay between the hydrogen bonding interactions and the free volumes, the former is responsible for antiplasticization whereas the latter leads to plasticization of the polymer materials. Further analysis confirms that the networks form hydrogen bonds with absorbed water molecules, which preferred to locate in the vicinity of polar groups on the polymer network, and water at higher concentration can cluster with each other. The diffusion coefficient increased with increasing water concentration; the same trend as the chan-e in fractional free volume but contrary to that in density. These results from our simulations were in good agreement with the general experimental observations and the free volume theory. (c) 2007 Elsevier Ltd. All rights reserved.