Gasification and pyrolysis are essential steps during thermal treatment or combustion of plastics. The rate of evolution of gaseous products affects all consecutive steps in combustion. Therefore, the knowledge of the kinetics of gasification of plastics is essential. A kinetic model for the gasification of plastics encompasses the rates of evolution of all gaseous pyrolysis products and the degradation of the polymer. During pyrolysis of polystyrene, high amounts of styrene are formed. However, the kinetic data for polystyrene pyrolysis available from the literature exhibit some inconsistency, and little information or kinetic data are available about the formation of higher aromatic compounds, for example, the styrene dimer and trimer. Therefore, in this work, the thermal degradation of polystyrene is investigated under isothermal conditions with the help of a gradient free reactor. Formal kinetic parameters for the overall degradation as well as the formation of dimer and trimer are determined. On the basis of these data, a detailed discussion of the mechanism is possible, and some elementary reactions are identified. Rate equations for the different species are formulated, and a kinetic model for the reaction rate of polymer degradation is evaluated. Furthermore, each rate coefficient of the model is determined or estimated.