The effects of hydration-dehydration cycling on the dynamic mechanical response of a rubber modified, mineral-filled epoxy resin based on the diglycidyl ether of bisphenol A cured with dicyandiamide are reported. Samples of the resin were aged in both deionized water and 5% w/w NaCl solution at 65 degreesC. A dynamic mechanical thermal analyzer was used to detect the changes in the mechanical spectrum of the material induced by moisture ingress. Water absorbed at weight fractions up to 4.3% caused plasticization of the polymer matrix, and the measured decrease in the glass transition temperature could be interpreted in terms of the polymer network-diluent theory. Water absorbed at levels >4.3% weight fraction resides in microcavities formed as result of the aging process and did not further affect the dynamic mechanical response of the resin. The dynamic mechanical properties of the material were independent of the previous history of exposure to humidity. In the presence of NaCl, the microcavity growth was prevented and the amount of water absorbed did not exceed 4.1% weight fraction.