A liquid diglycidyl ether of bisphenol A (DGEBA) epoxy resin is blended in various proportions with amine-terminated polyoxypropylene (POPTA) and cured using an aliphatic diamine hardener. The degree of crosslinking is varied by altering the ratio of diamine to epoxy molecules in the blend. The mixture undergoes almost complete phase separation during cure, forming spherical elastomer particles at POPTA concentrations up to 20 wt%, and a more co-continuous morphology at 25 wt %. In particulate blends, the highest toughness is achieved with nonstoichiometric amine-to-epoxy ratios, which produce low degrees of crosslinking in the resin phase. In these blends, the correlation between G(1C) and plateau modulus (above the resin T-g), over a wide range of amine-to-epoxy ratios, confirms the importance of resin ductility in determining the fracture resistance of rubber-modified thermosets.