In order to improve both the heat resistance and the toughness of cured epoxy resins, polyethersulfone (PES) was added to a biphenyl-type epoxy resin. The dispersion state of the PES in the epoxy resin was controlled by changing the molding temperature, and the effect of the morphologies on the heat resistance and toughness of the cured resins was investigated. In the resin with a homogeneous phase morphology, the glass transition temperature (T-g) of the resin containing 10 wt% PES increased about 20 degrees C, and the value of the fracture toughness also increased about 60% more than that of the unmodified resin. This is attributed to the formation of the semi interpenetrating polymer networks (semi-IPNs) composed of the epoxy network and linear PES. On the resin with phase separation structure, a significant increase in toughness was obtained only when the PES formed a continuous phase morphology. However, higher PES content is required to form the co-continuous phase morphology, which causes the increase in the viscosity of the resin. In addition, the T-g of this system was equal to or only slightly higher than that of the unmodified resin. (C) 2000 Elsevier Science Ltd. All rights reserved.