Impact modifiers in the form of rubber particles increase the toughness of styrene polymers and copolymers. It is thought that on impact cavitation in rubber particles occurs which reduces the detrimental dilatational stress in the bulk polymer without forming cracks in the brittle matrix or at the rubber-matrix interface. This void formation is thought to favour an increase of the energy dissipating deformation processes, namely by crazing in HIPS, where the voids are considered to ease craze formation, or by preferentially shear yielding in ABS at higher rubber concentration, where the voids support the plane stress situation which multiplies shear yielding. To test this model the cohesion strength and consequently the resistance to void formation of the rubber particles was increased by thermal cross-linking, or decreased by adding oil to the rubber particles which acts as nuclei for void formation. On annealing both the high impact polystyrene and rubber modified polystyrene-acrylonitrile copolymers embrittle. TEM of the fractured products showed that in the tough non-annealed products void formation in the rubber particles had occurred extensively, whereas after annealing this phenomena is hardly observed. The addition of oil can improve toughness by facilitating void formation. The reduction of voiding in the rubber particles of the annealed and more brittle products and toughening with oil as an additive to the rubber particles support the above proposed theory that void formation in rubber particles. increases the energy dissipation on impact and thus the toughness of the products.