Polymer degradation is an example of fragmentation of macromolecules of distributed molecular weight (MW). We investigated polystyrene (PS) oxidative degradation kinetics by di-tert-butyl peroxide in solvent trichlorobenzene at 428 K. Peroxide was consumed by the PS decomposition reaction and also by an independent deactivation reaction involving the polymer. Gel permeation chromatography (GPC) showed that the initial PS molar molecular weight distribution (MWD) was exponential in MW. After the peroxide was depleted (in less than 15 min), GPC analysis of polymer samples showed that the final MWD was also exponential in MW for all experimental conditions. The initial peroxide concentration, 5-40 g/l, and initial polymer concentration, 12.5-50 g/l, affected the reaction rates and hence the final MWDs. A continuous-distribution model for polymer fragmentation (first-order in both peroxide concentration and polymer MWD) and simultaneous peroxide deactivation (first-order in polymer concentration) describes the temporal behavior of the MWD and its moments. To be consistent with the experimental data, the model incorporates random chain scission and rate coefficients that increase linearly with MW. Moment analysis provides the ratio of the rate parameters for the peroxide deactivation and polymer decomposition.