Blends of a tetrafunctional epoxy resin and a thermosetting bismaleimide (BMI) resin cured with 4,4＇-diamino diphenyl sulfone were investigated. Information on the conversion of the reactive groups upon curing was obtained by Fourier transform infrared spectroscopy. Dynamic-mechanical tests carried out on both dry and wet samples indicated the formation of an IPN-like structure and a considerable reduction of the plasticizing effect of the water absorbed in the presence of BMI. Sorption measurements at 70 degrees C revealed a reduction of the equilibrium water uptake with respect to the neat epoxy matrix, and a slight increase of the apparent diffusion coefficient. Furthermore, the presence of BMI enhanced such properties as the flexural elastic modulus and the compressive yield stress while reducing slightly the fracture parameters K-lc, and G(lc) (critical stress intensity factor and critical strain energy release rate, respectively), as a consequence of the reduced ability of the IPN network to be plastically deformed under loading.