This paper describes water absorption in polydicyclopentadiene (PDCPD) and more specially the impact of thermal oxidation on its behavior. Unaged PDCPD is found to be almost hydrophobic consistently with its non-polar structure. During thermal oxidation, hydrophilicity progressively increases with ageing time. This increase is linked with the buildup of polar products (mainly ketones, carboxylic acids, hydroperoxides, alcohols). Moreover, it appears that water absorption in PDCPD obeys to Flory Huggins law for all conditions considered here. In the meantime, water diffusivity seems to decrease when oxidation level increases. Finally, a mastercurve that links carbonyl concentration, water diffusivity and water-polymer interaction parameter is proposed and allows changes of water resistance of PDCPD to be predicted.