Water diffusion through Unsaturated Polyester Resin (UPR) is investigated. From differential scanning calorimetry, we observe that the plasticization effect due to water sorption leads to large variations of T-g and is a reversible phenomenon. The transport of water through the polymer is characterized with a new instrument developed in our laboratory which shows that the water transport deviates from Fickian behavior. It is found that the diffusion coefficient is not a constant and experimental data are in good agreement with D = D(o)e(gamma)C where gamma is a plasticization coefficient.