Impact tests and fracture toughness tests using compact tension specimens were carried out on a number of slightly plasticized PVC compositions. These measurements, together with calculations from the craze thickness profile were used to determine the fracture mechanisms operating in the various tests. The marked decrease in the impact strength of PVC on addition of small amounts of a conventional plasticizer was found to be due to the plasticizer decreasing the stress intensity necessary to nucleate a craze at the notch tip of the impact specimen. The fracture toughness of the compact tension specimens which failed by a crazing mechanism increased with increasing plasticizer content. It is thought that the fracture of these specimens is controlled by the stress intensity necessary to propagate the pre-existing crack/craze system through the material. �� 1978 Chapman and Hall Ltd.