The present work investigates the effect on the morphology, fracture performances, and tensile properties of an epoxy polymer when modified with various percentages of a thermoplastic-polymeric toughener. The thermoplastic toughener was a poly(ether sulfone) copolymer with reactive end-groups. It was initially soluble in the epoxy-resin/hardener mixture but phase separated during the curing of the epoxy resin. After the epoxy had cured, the thermoplastic toughener, when present at relatively low concentrations, possessed a spherical-particulate morphology in an epoxy-rich continuous phase. However, as the weight percentage of the thermoplastic was increased the morphology changed to a co-continuous microstructure, and then to a phase-inverted microstructure of epoxy spherical particles in a thermoplastic-rich continuous phase. The Young's modulus and 0.2% proof stress of the epoxy polymer were relatively unaffected by the addition of the thermoplastic, whilst the ultimate tensile strength increased with increasing thermoplastic content. The fracture toughness and fracture energy of the formulations were found to increase steadily with increasing thermoplastic content. This increase was not, however, linked to the observed changes in morphology, but simply to the weight-percentage of the thermoplastic toughener added to the formulation.