The thermal decomposition and the glass transition temperature of poly p-methoxystyrene (PpOMeS) 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 polymer glass transition temperatures. Decomposition of PpOMeS under isothermal conditions during 50 min intervals at various temperatures or at a fixed temperature (320°C) during various intervals leads increasing quantities of high molecular weight material to yield low molecular weight products. Random scissions have been shown to break down the polymer chains which then volatilize via depolymeriza-tion. Activation energy for the decomposition of PpOMeS has been found to be less than that for the decomposition of polystyrene (PS). Variation of Tge(at q = l°K/min) withMnobeys the relation: Tge- (°K) = 386 - 4.67 × 105/Mn. The value of 386°K for the Tg∞ of PpOMeS is greater than the value (362°K) reported in the literature. A comparison of Tg∞ for PS (379°K) for poly p-methylstyrene (384°K) and for PpOMeS (386°K) reveals that the presence of p-CH3or p-OCH3groups neither influences the chain flexibility nor causes further steric hindrance than already exists in PS.