Degradation of polymeric matrix composite materials in marine environments has been experimentally investigated. Carbon/epoxy, carbon/bismaleimeide-epoxy, glass/epoxy and glass/PPS, and neat PPS were examined. The environments considered were distilled water and natural seawater at room temperature and 35°C. Water absorption was monitored during seven months of immersion. Several specimen geometries were investigated which allowed determination of longitudinal and transverse diffusivities. Degradation of mechanical performance due to water absorption was examined through transverse tension testing of dry and wet composites. The transverse modulus of the thermoset composites remained virtually unaffected by water absorption, while the modulus of glass/PPS dropped substantially, apparently due to extensive fiber/matrix debonding induced by water absorption. Transverse tensile strength was drastically reduced after water absorption for all composites. The composites degraded similarly in distilled water and seawater. Inspection of the fracture surfaces of dry and wet composites in a scanning electron microscope indicated that fiber/matrix adhesion was adequate in the dry composites, while in the wet composites numerous bare debonded fibers were observed, which substantiates that the reduction in transverse strength is largely due to fiber/matrix interfacial degradation.