The inherent brittleness of tetraglycidyl methylenedianiline (TGMDA) based epoxy formulations can be mitigated by the addition of carboxyl-terminated butadiene-nitrile rubbers (CTBNs) if piperidine is used as the curing agent. The problems of toxicity, reactivity and volatility associated with this reagent are overcome by using it in a ’latent’ form, as an amino alcohol or urea, after reacting it with a suitable epoxide or isocyanate. The fracture properties, measured by the critical strain energy release rate (G(IC)) and the critical stress intensity factor (K-Ic), of the new materials were compared to those of the TGMDA-diaminodiphenylsulfone (DDS) system. The best systems gave G(Ic) values up to 5.5 times those of the standard TGMDA-DDS formulation, while the addition of CTBN further increased this to up to 24 times; in comparison, addition of CTBN to the base formulation effected a threefold increase in GI,; there is however some lowering of the glass transition temperature and modulus of the modified resins. The morphology of the rubber-toughened materials, their reactivity, latency and water uptake are discussed.