The fracture toughness of polyvinylchloride (PVC) and PVC modified with 10% chlorinated polyethylene (PVC-CPE) was studied in vapour and in liquid environments by crack growth measurements on single-edge notch specimens under three-point bending at 23 degrees C. In addition, some results obtained in air at lower temperatures are presented. The fracture toughness is quantified by a stress intensity factor leading to failure after a given loading period. It is shown that for a given slow crack growth rate at 23 degrees C, the environment hardly affects the fracture toughness of PVC. In contrast, the slow crack growth in PVC-CPE at 23 degrees C is accelerated by the presence of benzene vapour, n-octane/benzene mixtures and gas condensate. A decrease in temperature results in an increase in fracture toughness, both for PVC and for PVC-CPE. A Dugdale model to describe the craze ahead of the crack was used to analyse the observed changes in fracture toughness.