An experimental study combined with theory based on distribution kinetics shows how polystyrene degradation is enhanced by continuous addition of peroxide. We investigated polystyrene oxidative degradation kinetics by di-tert-butyl peroxide in the solvent trichlorobenzene at temperatures from 100 to 150 degrees C. The basic experimental data consisted of the time dependence of the polystyrene molecular-weight distribution (MWD). The model. accounts for the rate of peroxide dissociation to radicals that rapidly abstract hydrogen from the polymer backbone. The model also includes random hydrogen transfer among polymer molecules and their radicals, and describes p-scission of radicals. For the case that all polymer rate coefficients increase linearly with MW, an analytical (similarity) solution is formulated for the MWD. Experimental data support the general form of the model results, including the Induction time for peroxide decomposition, and provide estimates of rates for peroxide dissociation, hydrogen abstraction, and random scission. Rate coefficient temperature dependences yield activation energies.