A detailed study of the kinetics of environmental degradation in graphite-epoxy composites show a close correlation between analytical predictions and experimentally observed changes in interlaminar shear and fracture energy response under high moisture exposure conditions. Unaged composite specimens exhibit high interlaminar shear strength λ_b ⩾ 850 kg/cm² (12 000 psi) and relatively low fracture energy W_b/A ≅ 10 to 20 kg cm/cm² (56 to 112 lb in./in.²). Exposure to 95 percent relative humidity or water immersion at 100°C (212°F) for times t ⩾ 200 h produces a 30 to 50 percent reduction in λ_b accompanied by a concurrent two to fivefold increase in W_b/A and acoustic energy absorption. These property changes are shown to be irreversible and directly related to cumulative moisture degradation of the fiber-matrix interfacial bond. The magnitudes of these property changes are consistent with surface energy analysis and micromechanics predictions.