Most research on polymer degradation is for single polymers, even though the thermal decomposition of polymer mixtures is of interest both practically and theoretically. Polymer degradation rates depend on the mixture type, and adding a polymer can increase, decrease, or leave unchanged the degradation rate of the first polymer. We show how distribution kinetics theory, based on molecular weight distributions (MWDs), provides expressions for such degradation rates of binary polymer mixtures. The approach accounts for initiation, termination, hydrogen abstraction, and radical-chain scission in the governing equations for MWDs. Molecular weight moments yield expressions for molar and mass concentrations and rate coefficients for combinations of random-chain and chain-end scission. Experimental data show the concentration effect of poly(α-methylstyrene) (PAMS) on the degradation of polystyrene dissolved in mineral oil at 275 °C in a batch reactor. PAMS and polystyrene undergo mainly chain-end and random-chain scission, respectively. Samples analyzed by gel permeation chromatography yielded the time evolution of the MWD. The results indicated that, because of the interaction of mixed radicals with polymer by hydrogen abstraction, the polystyrene degradation rate decreases with increasing PAMS concentration.