The microstructure and properties of two epoxy-resin systems which have been modified with varying amounts of a thermoplastic to improve the toughness of the thermosetting epoxy polymers, have been studied. The curing agent was 4,4’-diaminodiphenylsulphone and the thermoplastic was a reactively terminated poly(ether sulphone) copolymer. Different microstructures were found to occur as the concentration of the thermoplastic component was steadily increased. In particular, the relationships between the microstructures and values of stress-intensity factor, K(Ic), and fracture energy, G(Ic) were explored.