Disordered glass microsphere-epoxy composites have been used in a study of diffusional, electrical and mechanical effects of interfaces in polymer-matrix composites exposed to pure water. Mass gain measurements on composites manufactured from 10 μm silane-treated microspheres indicate initial near-Fickian diffusion with water saturation times on the order of 500 h. However, electrical measurements indicate water transport at rates at least 100 times more rapid. This behaviour is interpreted in terms of a cellular microstructure with areas of low cross-link density separating highly cross-linked areas. Rapid water transport can thus occur in areas of low cross-linking, even without the contribution of connected clusters of particles where rapid interfacial water transport occurs substantially ahead of the main diffusion front. Reductions in ultimate tensile strength and fracture energy in dry and water-saturated tensile test specimens are observed with increasing volume fraction of glass spheres but with a distinct plateau between about 6% and 12% Vf. This can be explained in terms of secondary cracking below the percolation threshold which causes toughening of the composite. However, a few % above pc (≍ 16%), most particles belong to the percolating cluster and the primary crack can grow without hindrance.