The effect of temperature on moisture absorption and hygrothermal aging in a commercial bismaleimide (BMI) resin and its composites are studied. The resin and composites display a two-stage diffusion behavior, With the first and second stages being diffusion- and relaxation-controlled, respectively. An increase in temperature accelerates moisture absorption in both the first and second stages. The activation energy of diffusion is very similar in the neat resin and composites, which suggests minimal interface effect on short term moisture diffusion. The equilibrium uptake of the diffusion-controlled first stage decreases with increasing temperature, indicating moisture absorption in the BMT resin is exothermic. The heat of absorption calculated from the temperature dependence of the quasi-equilibrium uptake is on the order of -3 kJ/mol. Although moisture absorption at relatively low temperatures is dominated by relaxation of the resin, significant chemical degradation occurs at 90 degreesC. The chemical degradation most likely involves the hydrolysis of imide units, resulting in depolymerization and chain scission.