It was the object of the present article to discuss the effect of the addition of a tetrafunctional resin, namely, tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM), and of a novel 40:60 polyethersulfone:polyetherethersulfone (PES:PEES) copolymer on a blend constituted by a difunctional resin, diglycidyl ether of bisphenol S (DGEBS), and an aromatic ammine, 4,4'-diaminodiphenylsulfone (4,4'DDS). The formulated blends were fully characterized in terms of the morphology and the viscoelastic and mechanical properties. The modification by the tetrafunctional resin was proved to be a useful means to improve the glass transition temperature and, therefore, to widen the range of the working temperature. The addition of the novel copolymer resulted in a marked increase in toughness with out negatively affecting the thermal and elastic properties of the blend. TEM analysis, despite the high amount of thermoplastic used, showed a homogeneous morphology that would lead to the conclusion of the existence of a full-IPN network. Moisture-uptake experiments were conducted both on the blend and on the resins on their own. It was shown that the combination of the difunctional resin with the tetrafunctional led to a decrease of the percent of the water absorbed. (C) 2003 Wiley Periodicals, Inc.