The thermal decomposition and the glass transition temperature of poly(p-chlorostyrene) (PpCIS) were studied with a Model 2 differential scanning calorimeter (DSC). The undecom-posed and decomposed polymers were analyzed by gel permeation chromatography for molecular weight distributions and by DSC for changes in the polymer glass transition temperature. The decomposition of PpCIS under isothermal conditions during 50 min intervals at various temperatures or at a fixed temperature (320°C) but for different periods is characterized by the disappearance of increasing quantities of high molecular weight polymer and the appearance of low molecular weight products. Random scissions have been shown to break down the polymer chains which depolymerize into volatile products. Activation energy (72 kcal/mole) for the decomposition of PpCIS is lower than that (103 kcal/mole) for the decomposition of polystyrene.