AbstractAbsorbed moisture was found to plasticize the type of cross‐linked epoxy resin system investigated, causing a lowering of the Ta and in turn affecting mechanical response, such as by shifting the relaxation moduli to shorter times. The phenomenon is described and quantified in terms of an existing free volume theory relationship. Weight gains greater than equilibrium amounts were observed without accompanyingTgchanges which is attributed to moisture entrapment during microcracking in the resin. Creep studies performed under water at 300° F established an additional mechanism for loss of elevated temperature properties which is characterized by the appearance and growth of cracks in the material. The observed failure processes are described in terms of an existing theory for rupture of cross‐linked rubbers. The process is governed by the synergistic effects of moisture, temperature, and stress. Moisture changes the viscoelastic response of the material so that stress‐induced crack formation and growth is facil