Abstract - The thermal decomposition and combustion mechanisms of PVC are examined. With respect to thermal decomposition, the results of a deuterium labeling (d-PVC) study indicate that the mechanism of benzene formation involves an intramolecular cyclization step rather than an intermolecular Diels-Alder condensation step. With respect to combustion, the evolution and ultimate fate of hydrogen chloride and other combustion gases generated under NBS Smoke Density Chamber conditions is described. The combustion of rigid PVC, flexible PVC and rigid PVC-wood mixtures has been carried out. It was observed that under smoldering conditions, rigid PVC evolves combustion gases in a sequence that agrees with the current mechanism of decomposition. It was discovered that under flaming conditions, the concentration of hydrogen chloride in the NBS Smoke Density Chamber rapidly decreases and that the decay follows first order kinetics. Analytical data is presented which shows that this decay is due to HCl condensation on the chamber walls and is not an experimental artifact. The data from these experiments indicate that water generated during combustion plays a key role in this condensation. Recent data published on the exposure of test animals to the combustion gases from PVC are reviewed in light of the above findings.