Three aspects of the rheological behavior of PVC are considered. These are (a) particulate flow, which occurs below the true crystal-line melting point, (b) true melting flow, which occurs at or above the melting point, and (c) the flow of PVC compounds. The effect of structural differences resulting from different polymerization temperatures employed in preparing different molecular weight polymers on the melt viscosity and flow activation energy is re-examined, and new data a r e presented to unconfound the effect of syndiotacticity and molecular weight. The three types of flow behavior of PVC are shown to be represented by three distinct flow regions. Because of thermal instability, the true melt flow region is achieved only with difficulty. However, in commercial operations this latter state is seldom, i f ever, achieved in rigid formulations. The effect of compounding additives, especially lubricants and plasticizers, and the importance of thermal and mastication history on the flow behavior are discussed with reference to the flow mechanism.