Test data is described for polycarbonate single edge notch specimens at a range of loading rates and two thicknesses at 20°C. The transition from ductile to brittle failure is shown to be both rate and thickness dependent. The effects may be described in terms of a plane stress and a plane strain fracture toughness coupled with a rate dependent yield stress and a plastic zone size to thickness ratio criterion. An increasing percentage of the incidence of brittle failure with rate rather than a sharp transition is noted and explained in terms of a distribution of toughness values introduced by variations in the notching method.