The moisture diffusion process of an epoxy system is studied as a function of epoxy-amine stoichiometry and the resulting microstructure. Differences in diffusion behavior are related to the relative importance of diffusion through the low-density and high-density microstructural. phases for different stoichiometries. Also, changes in saturation level with stoichiometry are explained by competing effects of free volume versus the content of the low-density phase. Increasing the humidity level causes a corresponding increase in saturation level, while increasing the temperature causes more pronounced non-Fickian behavior. The effects of absorbed moisture on the thermomechanical properties of the epoxies are also investigated. Reductions in the glass transition temperature, T-g, and moisture-induced swelling strains are measured after exposure of samples to the three conditioning environments. Moisture-induced swelling strains increase with increasing moisture content. The reductions in T-g range from 5 to 20 degrees C and are generally larger for amine-rich samples than for epoxy-rich and stoichiometric samples.