The thermal degradation of polyvinyl chloride is primarily a dehydrochlorination reaction. In an atmosphere of N₂ the rate tends to approach a steady state and is not affected by a build-up of HCl. A drop in intrinsic viscosity in the early stages of degradation followed by an increase, indicates that chain scission and cross-linking are competing reactions in the presence of O₂. Heavy metal stabilisers absorb HCl in a N₂ atmosphere, but when O₂ is present they appear to function through some other mechanism. Exposure to ultra-violet radiation at room temp. results in the evolution of small amounts of HCl. Measurable quantities of O₂ are taken up during this process. The irradiated polymer is more susceptible to thermal degradation than the original. Possible mechan isms for these processes are discussed. -- AATA