An experimental investigation was undertaken to determine the effects of marine environmental exposure on the mechanical properties of vinylester resins (510A and 8084) and unidirectional carbon fiber/VE510A composites. Carbon fibers are inert to water and vinylester resins absorb small amounts of moisture. Such materials are of great interest for Naval ship structures.  The effect of carbon fiber sizing (F and G sizings) on the composite strengths was also examined. Neat resin and composite specimens were exposed to seawater until they approached saturation. An analysis of moisture absorption of the composites was performed which shows that moisture up-take is dominated by the fiber/matrix region which absorbs up to 90% of the total moisture. Dry and moisture saturated neat resin and composite specimens were tested in tension, compression, and shear. Composites with F-sized carbon fibers were overall stronger than those with G-sized fibers at both dry and moisture saturated conditions. Unidirectional composite specimens experienced moderate reductions of the strengths governed by the fiber/matrix interface after moisture absorption. The type of fiber sizing was found to have a strong effect on the fiber/matrix interface dominated strengths.