An epoxy resin, cured using an anhydride hardener, has been modified by the addition of preformed core-shell rubber (CSR) particles which were approximately 100 or 300 nm in diameter. The glass transition temperature, T g, of the cured epoxy polymer was 145 °C. Microscopy showed that the CSR particles were well dispersed through the epoxy matrix. The Young's modulus and tensile strength were reduced, and the glass transition temperature of the epoxy was unchanged by the addition of the CSR particles. The fracture energy increased from 77 J/m2 for the unmodified epoxy to 840 J/m2 for the epoxy with 15 wt% of 100-nm diameter CSR particles. The measured fracture energies were compared to those using a similar amount of carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber. The CTBN particles provided a larger toughening effect when compared to CSR particles, but reduced the glass transition temperature of the epoxy. For the CSR-modified epoxies, the toughening mechanisms were identified using scanning electron microscopy of the fracture surfaces. Debonding of the cores of the CSR particles from the shells was observed, accompanied by plastic void growth of the epoxy and shell. The observed mechanisms of shear band yielding and plastic void growth were modelled using the Hsieh et al. approach (J Mater Sci 45:1193-1210). Excellent agreement between the experimental and the predicted fracture energies was found. This analysis showed that the major toughening mechanism, responsible for 80-90% of the increase in fracture energy, was the plastic void growth.