Highly crosslinked, brittle epoxy resin based on tètraglycidyl-4,4′-diaminodiphenyl methane (TGDDM) cured with 4,4′-diaminodiphenyl sulphone (DDS) is widely used as a matrix material for aerospace composite applications. This work describes how inclusion of a polyetherimide (PEI) thermoplastic rich phase can significantly toughen the resin without a fall in apparent stiffness, which accompanies the more traditional method of rubber toughening. Dynamic mechanical analysis and scanning electron microscopy are used to characterize morphology, while fracture toughness and flexural modulus measurements are also performed on the cured resins. The extent of toughening is found to be dependent upon PEI concentration and test temperature. It is observed that the PEI rich phase toughens through ductile drawing across the faces of the advancing crack tip, and that this is the dominant toughening mechanism in the material.