The thermal decomposition and the glass transition temperature of poly(p-bromostyrene) (PpBrS) were studied with a (DSC-2) differential scanning calorimeter. The undecomposed and decomposed polymers were analyzed by gel permeation chromatography for molecular weight distributions and by DSC-2 for changes in the glass transition temperatures. The decomposition of PpBrS under isothermal conditions during 50 min intervals at increasing temperatures or at a fixed temperature (320°C) for increasing periods results in increasing quantities of high molecular weight polymer yielding greater amounts of low molecular weight products. Random scissions have been shown to break the polymer chains which then volatilize via depolymerization. Activation energy for the decomposition of PpBrS at 118 kcal/mol is near that (102 kcal/mol) of polystyrene (PS). Variation of Tge(at q = 1 K/min) withMobeys the relation