A detailed characterisation of the high speed fracture behaviour of polycarbonate/core-shell impact modifier blends using thick (6.25 mm) injection moulded bars with sharp notches via the Vu-Khanh methodology and data analysis is described. This expands on a previous report that examined the effect of polycarbonate molecular weight and melt blending conditions on the customary stress-strain and impact properties of such blends. Load-deflection curves and impact strength as a function of temperature are also reported. Morphological features near crack tips formed at high speed were examined by microscopy to gain insight about the sequence of events that occur during crack propagation. The use of thick specimens with sharp notches gives a better discrimination of how the concentration and degree of dispersion of impact modifier influence the toughness of these polycarbonate blends than does Izod testing using thin specimens with standard notches. High molecular weight of the matrix and good dispersion of the impact modifier in the matrix lead to both higher fracture energy at initiation and tearing modulus. Model calculations are presented to explain the different fracture responses under plane strain versus plane stress conditions caused by addition of impact modifier to polycarbonate.