Four samples of PVC polymerized in suspension (S) or emulsion (E) or in bulk at two different temperatures (Mi and M2) have been treated by ozone between -20 and 120°C, either in the solid state or in tetrachloroethane solution. Chain scission and peroxidation are observed with a clear parallel between the two processes. The polymer which contains the smaller amount of internal double bond (broken down at -20°C) is less sensitive to peroxidation and further chain scission at higher temperatures. Chemical or thermal peroxide decomposition experiments show that there are no residual peroxide structures in the backbone of the chain and the presence of ozone is necessary to cause chain scission. Ozonization in the solid state shows that, under Tg, the peroxidation process involves a dehydrochlorination step. A tentative mechanism for the ozonization process is suggested.