An epoxy resin based on diglycidyl ether of bisphenol A has been modified with poly(ether sulfone) (PES) and poly(acrylonitrile-co-butadiene) rubber (NBR) and cured with 4,4’-diaminodiphenylsulfone. The fracture behaviour of the ternary blend was examined using a fracture mechanics approach. Depending on the combination of PES and NBR, two distinct morphological states were shown, and the fracture behaviour varied from brittle to ductile with changing morphology. From fracture surface observations by scanning electron microscopy, the ternary blends that showed ductile fracture behaviour have a phase-inverted morphology in which the NBR-rich phase formed a continuous matrix, the epoxy-rich phase formed particles and the PES-rich phase also formed particles around the epoxy particles. Dynamic mechanical thermal analysis revealed that the ternary blends exhibited high T-g, as high as the T-g of the unmodified epoxy resin.