An investigation was carried out into the effect of an additive called “fortifier” on curing, morphology, and thermomechanical properties of epoxy resins. Differential scanning calorimetry, electron microscopy, and a series of mechanical measurements were performed on bifunctional and tetrafunctional formulations. It was found that the addition of fortifier resulted in an increase in the fracture surface roughness, the size of morphological inhomogeneities observed in the fractographs, and the ultimate tensile strength. At the same time, the glass transition temperature was found to decrease by at least 25°C in going from 0 to 15 phr of the fortifier in a formulation. The presence of fortifier had little effect on the rate of cure and on the strain energy release rate.