The introduction of rubbery particles can be applied to enhance shear yielding and, consequently, the toughness of brittle amorphous polymers. The critical transition in these polymers from crazing to shear yielding requires a submicrometer- or even nanometer-sized rubbery phase. These can be obtained via coalescence suppression in processes involving chemically induced phase separation but are also obtained in interpenetrating polymer networks (IPN) where cross-linking or gelation is responsible for the morphology control. In all cases, the formation of a nanometer-sized morphology is accompanied by an enhanced interphase mixing, i.e., incomplete demixing resulting in a broad interface in which the composition gradually changes from one phase to the other. In this study, the influence of interphase mixing on the mechanical properties has been investigated. Besides the standard semi-IPN system based on poly(methyl methacrylate) and aliphatic epoxy resins, two additional systems being composed of the same constituents but with an increased degree of interfacial mixing have been investigated: a full-IPN prepared by cross-linking the acrylate phase and a copolymer system in which the acrylate phase is chemically bonded to the epoxy phase. In situ small-angle X-ray scattering experiments during tensile deformation demonstrated that the microscopic deformation mechanism is clearly influenced by the degree of demixing. Despite this, the macroscopic toughness is found to be rather system independent since for all three systems, a comparable synergistic toughening effect is observed in both tensile and impact deformation.