In the title of this paper the term "hydrothermal", meaning separate or combined conditions of high moisture and temperature, is introduced to describe the complex property degradation processes in two graphite-epoxy composites. It became evident in the course of this study that important hydroelastic stresses degrade composite strength in much the same fashion that thermoelastic stresses do. In fact, the study shows that there are complex internal stress effects within the composite, probably concentrated at the fiber-matrix interface, which depend upon the detailed prior history of moisture-temperature exposure .