Rheological, thermomechanical, and morphological properties of blends of two different epoxy resins with hydroxyl functionalized hyperbranched polymers have been studied. The hydroxyl functionalized hyperbranched polymers used in this work had different generation number and different number of terminal groups. The difunctional epoxy resin has been also mixed with a linear aromatic thermoplastic in order to compare its effects with that of the hyperbranched polymers. All the blends have been characterized by linear elastic fracture mechanics (LEFM) testing to evaluate the efficiency of the two types of toughening modifiers. In addition, the water uptake has been evaluated for all the formulations. (C) 2005 Society of Plastics Engineers.