Fracture of rubber-toughened polystyrene (HIPS) has been studied via post-mortem analysis of specimens with arrested cracks formed under impact loading conditions. This methodology involves the study of the deformed region around an arrested crack tip by electron microscopy. It was applied to two commercial HIPS products and two experimental products with sub-micron rubber particles. For the HIPS materials, the arrested crack tip has a blunted shape with a width from 50 to 100 mu m depending of the material and its impact resistance. As expected, massive crazing was found in all these materials around the crack. In a narrow zone just ahead of the crack tip, where the crack would have continued to grow if not arrested, considerable elongation and deformation of the rubber particles was observed. This zone appears to be the precursor of the crack. In this region, "dilatation" and "shear" bands of more than 50 mu m long were observed. Higher magnification TEM observations show highly deformed rubber particles ("S" or "tear-drop" shape) along the shear bands of 1-6 mu m width. Three different kind of cavitation was observed just ahead of the crack tip: (1) small voids inside the membranes of the rubber particles; (2) large voids in the interface between rubber particles and the matrix and (3) interfacial cavities within the particles between the rubber membrane and the polystyrene occlusions. Within the narrow shear-bands there is evidence of shear yielding in the fracture of HIPS, similar to that observed in more ductile polymers. (C) 2000 Elsevier Science Ltd. All rights reserved.