The thermal degradation of polystyrene (PS) was studied in the presence of an acid catalyst, p-toluene sulfonic acid (p-TSA). The degradation was carried out at various temperatures (150-170°C) and molar concentrations of the acid. The reaction samples were analyzed by gel permeation chromatography (GPC) and the molecular weight distribution (MWD) was obtained as a function of time. Continuous distribution kinetics were employed with a stoichiometric kernel for random chain scission to determine the rate coefficients. The degradation rate was found to increase considerably in presence of the acid catalyst, and the theory, based on radical mechanisms, satisfactorily explains the experimental data. The activation energy, determined from the dependence of the degradation rate coefficient on temperature, was 22 kcal/mol. © 2002 Elsevier Science Ltd. All rights reserved.