Wood-plastic composites (WPC) were prepared with two monomers, methylmethacrylate (MMA) and butylmethacrylate (BMA), using ionizing radiation and two low-grade woods (simul and mango). Moisture adsorption characteristics of both wood and WPC were investigated. The equilibrium moisture contents of WPC were lower than the untreated wood in the entire water activity range. The experimental sorption data were fitted to the Henderson, Chung-Pfost, Smith, and Oswin models. Henderson's model gave the best prediction of the adsorption of wood and WPC.