The kinetics of the oxidative degradation of polystyrene in dichlorobenzene in the presence of benzoyl peroxide was studied in a microwave reactor. Solutions with peroxide concentration ranging from 15 to 40 kg/m3 were irradiated by microwave of 700 W and frequency of 2.45 MHz with a different cycle heating period ranging from 20 to 45 s. The molecular-weight distributions were measured as a function of reaction time by gel permeation chromatography. Because the temperature of the system continuously varies with reaction time, a radical mechanism for oxidative degradation based on continuous distribution kinetics with time/temperature-dependent rate coefficients was proposed. The activation energy of 46.4 kJ/mol for the microwave-assisted oxidative degradation of polystyrene was obtained by nonlinearly regressing the experimental data.