The hydrothermal ageing of an epoxy thermoset modified with different amounts of poly(vinyl acetate), PVAc, was studied by gravimetric analysis, dynamic mechanical thermal analysis, and scanning electron microscopy. Differences in the water uptake process were observed as a function of the PVAc content, these differences were related to the morphology. Samples with nodular morphology, having spheres of thermoplastic dispersed in the epoxy matrix, showed water absorption behavior similar to neat epoxy. However, samples with inverted morphology, in which the thermoplastic formed the continuous phase, were more susceptible to water absorption; they exhibited a continued water uptake that could be described by a two-stage diffusion model. Samples with inverted morphology showed debonding between phases after hydrothermal ageing. Different dynamic-mechanical behavior has been found for samples that had epoxy matrix and samples with inverted morphology, both before and after hydrothermal ageing.