Results of experiments to evaluate the degree of moisture-induced swelling and plasticization in T300/934, T300/5209, GY70/339 composites are described. Swelling was found to be a nonlinear reversible function of equilibrium moisture content. Out-of-plane swelling was several times larger than in-plane transverse swelling in the unidirectional composites. Equilibrium moisture content was found to vary with exposure temperature as well as humidity in the 339 and 5209 matrix systems. The in-plane shear properties were determined as a function of time, temperature, and equilibrium moisture content, and master curves of temperature and moisture-compensated modulus versus time were constructed for each material. Nonsymmetric (04/904)T cross-plied laminates were employed to study the alteration of residual stresses in T300/5209 and GY70/339 as a function of temperature and moisture content. The level of residual stresses measured for a given equilibrium moisture content was found to be a function of the temperature at which the moisture was absorbed.