The effect of hydrothermal cycling on the water absorption behaviour of a dicyandiamide cured silica filled rubber modified epoxy resin system is reported. The water absorption and desorption behaviour of the resin were monitored gravimetrically. Dielectric and dynamic mechanical thermal analyses were carried out on samples, selected from the batches being exposed, at times which reflect various critical points in the ageing profile. It was observed that although the saturation water uptake for the samples was independent of the hydrothermal history; the equilibrium value of the mass on desorption did depend on the hydrothermal history. Deviations from simple Fickian behaviour were observed for samples, which had undergone a previous absorption-desorption cycle. This phenomenon is interpreted as water being absorbed in the polar rubbery phase of the material and not readily released on dehydration. The dielectric data provide a clear indication of the presence of a heterophase structure in the epoxy resin and are consistent with the postulate that, whilst the initial water absorption occurs reversibly in the epoxy matrix, subsequent absorption of water into the polar rubbery phase is irreversible at 50 degrees C, the temperature used in the study.