Toughness enhancement of brittle amorphous polymers can be obtained via microstructural adjustments, resulting effectively in a removal of intrinsic strain softening on the mesoscale via a decrease in yield stress and an increase in strain hardening. Generally the introduction of a dispersed soft phase is required with an optimum size that tends to be in the range of several tens of nanometers rather than microns. This morphology cannot be realized via conventional processing techniques like physical blending, and in this study, chemically induced phase separation is used as a route for obtaining a fine dispersion of rubbery particles in polystyrene (PS) and poly(methyl methacrylate) (PMMA). In the case of simultaneously polymerized MMA/aliphatic-epoxy systems tensile toughening is observed at moderate (rubbery) epoxy volume fractions during slow speed deformation. Once the deformation speed is increased to impact conditions, the material, however, responds again in a brittle manner. The observations have been that predeformation of the samples at a low deformation rate, prior to impact testing, precavitates the specimens and regenerates toughness. This indicates that-apart from the obvious influence of the microstructure-the properties of the dispersed phase are of decisive importance.