Nitrated polyetherimide (NI-PEI) was obtained by chemically modifying a commercially available polyetherimide (PEI) and was characterized by nuclear magnetic resonance and infra-red spectroscopies, elemental analysis and viscometry. NI-PEI and NI-PEI/PEI mixtures were blended with an epoxy resin (MY0510) which was cured with 3,3’-diaminodiphenolsulfone. The morphology and fracture behaviour of these blends were examined by dynamic mechanical thermal analysis and by a three-point bending fracture test. A two-phase morphology exists with substantial toughening occurring at PEI and NI-PEI concentrations of 5 and 10 wt%.