A ductile thermoplastic, polycarbonate, has been blended with the diglycidyl ether of bisphenol A (DGEBA) epoxy resin to increase the ’toughenability’ of the resin. Cured epoxy-polycarbonate systems showed no increase in fracture toughness relative to the neat epoxy resin. The binary blends were miscible, single-phase systems. Addition of rubber to both the epoxy and the epoxy-polycarbonate blends leads to significant improvements in the critical strain energy release rate, G(IC), of the cured resin. The presence of the polycarbonate produced no additional improvement in G(IC) relative to rubber toughening alone. This is ascribed to a degradation reaction of the polycarbonate occurring during blending. The chemical nature of this reaction is discussed.