Our preliminary investigations indicate that recycled rubber crumb may be used as a toughening agent for brittle thermoplastic such as polystyrene. The toughness of the resultant materials was found to increase with increasing rubber-to-matrix adhesion and decreasing particle size. Although crazing is expected to play some role in the toughening mechanism, the absence of a yield point and the type of fracture morphology indicate it is only a minor factor It is suggested that the main contribution is from a crack bridging mechanism. The increased energy to break results from the rupture of the rubber particles. Further studies in which crumb with known tear strength and tensile properties is used as a filler are currently being carried out. It is hoped that this will lead to a clearer understanding of the toughening mechanism.