An experimental investigation was undertaken to determine the effects of moisture on the mechanical properties of vinylester resins (510A and 8084). The resins were exposed to seawater at 40°C and humid air at 50°C until the moisture content equilibrated. Both resins absorbed small amounts of moisture. Mechanical testing of both resins at dry and saturated conditions revealed very brittle behavior under tension and shear loading. The 8084 resin is more ductile than 510A at dry and saturated conditions. Moisture absorption was found to only slightly influence the mechanical response of these resins. Dry shear test specimens failed at quite low shear stresses in a resolved tensile mode and displayed sensitivity to the stress concentration in the Iosipescu shear specimen. Moisture absorption slightly enhanced ductility and reduced notch sensitivity which resulted in increased shear strength. POLYM. ENG. SCI., 53:2413–2421, 2013. ©2013 Society of Plastics Engineers