Disposal of plastic waste is a worldwide problem that has prompted investigation of plastics recycling, including thermolysis methods. Most basic research on degradation, however, is for single polymers. Waste streams usually contain mixtures of polymers and it may be costly to separate them prior to degradation. Degradation rates depend on the mixture type, and adding another polymer can increase, decrease, or leave unchanged the degradation rate of the first polymer. Determining the decomposition mechanisms for polymer mixtures is of great interest for polymer recycling. In this study, we present techniques to determine the degradation kinetics of solubilized binary polymer mixtures by examining the time evolution of molecular-weight distributions (MWDs). Because the reaction mechanism for polymer degradation involves radicals, we have developed an approach that accounts for the elementary reactions of initiation, termination, hydrogen abstraction, and chain scission. We determined the concentration effect of poly(a-methyl styrene) (PAMS) on the random-chain degradation of polystyrene dissolved in mineral oil at 275 °C in a batch reactor by evaluating the time evolution of the MWDs. Molecular-weight moments yielded expressions for the number- and weight-average MW and degradation rate coefficients. The experimental data indicated that the interaction of mixed radicals with polymer by hydrogen abstraction caused the random-chain scission degradation rate of polystyrene to decrease with increasing PAMS concentration.