An experimental study showed the enhancement effect of di-tert-butyl peroxide on poly(alpha -methylstyrene) (PAMS) thermal degradation in trichlorobenzene solution (12.5 g/L). Peroxide was continuously injected into the reacting polymer solution at low flow rates (0.3-4.0 mL/h). Samples were taken periodically and analyzed by GPC to determine the temporal behavior of the molecular weight distribution. Peroxide addition fundamentally altered the degradation mechanism to one in which random degradation was significant. A distribution kinetics model based on the fundamental free-radical processes of initiation -termination, beta -scission, and hydrogen abstraction was developed to describe the irreversible polymer degradation process. Reaction rate coefficients that are linearly dependent on the polymer molecular weight are the model parameters fitted to experimental data. The Arrhenius activation energies for individual steps in the reaction mechanism were calculated on the basis of the temperature dependence of the reaction rate. The different mechanisms for peroxide-enhanced thermolysis of PAMS and polystyrene are clarified by the distribution kinetics approach.