The deformation behaviour of ethylene-propylene-diene monomer rubber-modified polycarbonate blends (PC/EPDM; where the EPDM is adhering to the PC matrix) is studied and compared with non-adhering core-shell rubber-modified polycarbonate blends (PC/CS) using slow-speed uniaxial tensile testing and notched high-speed tensile testing. As expected, only the latter testing method reveals the existence of the critical thickness of PC as a function of temperature. At a constant ligament thickness the brittle-to-tough transition temperature (T(BT)) strongly increases upon replacing a hole (i.e. non-adhering rubber) by adhering EPDM rubbery particles. Electron beam irradiation is applied to selectively crosslink the dispersed EPDM rubbery phase and, consequently, increase its cavitation stress (which is proportional to the dynamic shear modulus). The T(BT) proved to be highly influenced by the thus modified cavitation stress of the dispersed phase. In terms of the energy-based criterion for the prediction of the critical thickness, this can only qualitatively be explained by (i) an increased strain energy density and/or by (ii) a change in the matrix volume involved in supplying the stored elastic energy.