Graphite -and boron-fiber-reinforced composites, as well as castings of current resin systems, are evaluated to determine the effects of moisture andor high humidity on their physical properties and their room and elevated temperature mechanical properties. All of the neat resin castings are found to absorb moisture and swell. Associated with moisture absorption is a loss in elevated temperature tensile strength. All of the composite systems show weight gains and thickness increases when subjected to a high humidity environment. However, the effect of absorbed moisture on the elevated temperature mechanical properties of composites is determined principally by the lay-up of the laminate andor the test being applied. For both castings and composites the effects of moisture are found to be reversible.