This paper presents results for wet aging of five glass fiber-reinforced polymer matrix composites. Specimens were immersed in distilled water for 2 years at four temperatures (5, 20, 40, and 60°C), at both atmospheric pressure and 10 MPa hydrostatic pressure. Absorption kinetics and the evolution of properties were followed through periodic Tg measurements, modal analysis, and destructive mechanical testing. There is no direct correlation between the quantity of water absorbed and the loss of mechanical properties; this emphasizes that absorption curves are of limited use for material selection. A hydrostatic pressure of 10 MPa accelerates moisture absorption in the epoxy composites. The establishment of a large data base of results has enabled accelerated test methods to be critically evaluated and the role of temperature and pressure as accelerating parameters is discussed.