The effect of the viscosity of the epoxy prepolymer on the generated morphology in a rubber-toughened epoxy system was investigated as a function of the molecular weight of the epoxy prepolymer and the curing temperature. The formulations were composed of diglycidyl ether of bisphenol A and 4,4’-diaminodiphenylsulfone, in the presence of esterified carboxy-terminated butadiene-acrylonitrile random copolymer. The cure reaction was described by an autocatalytic reaction mechanism. Parameters describing the generated morphology, i.e. average diameter of particles, volume fraction of dispersed phase and concentration of dispersed-phase particles, volume fraction of dispersed phase and concentration of dispersed-phase particles, were determined from scanning electron microscope observations. Tt was observed that the morphology changes from a bimodal to a unimodal distribution with curing temperature. This phenomenon was schematically explained by the location of the trajectory that the evolving matrix composition follows through the metastable region.