Water absorption and thermornechanical properties of epoxy systems based on multifunctional dicyclopentadiene epoxy novolac resin Tactix556 cured with 4,4' diaminodiphenilsulfone (4,4'DDS) as curing agent has been studied. The base system was modified by the addition of a novel 40 : 60 PES: PEES (Polyethersulphone: Polyetheretheresulphone) amine-ended copolymer to improve toughness properties. The effect of thermoplastic addition on water adsorption was studied by gravimetric experiments. The viscoelastic properties of the resulting blend were analyzed by means of dynamic mechanical thermal analysis. The formulated systems were compared with a system based on tetraglycidyl-4,4'diaminodiplienylmethane resin (MY721.) cured with 4,4' diaminodiphenilsulfone. The use of Tactix556 resin showed that water uptake values were minimized while retaining high glass transition temperatures, and toughness values were found in the same range of standard toughened matrices used for aerospace composites. (c) 2006 Wiley Periodicals, Inc.