Thermal degradation and gasification are essential steps during thermal treatment or combustion of plastics. The thermal degradation of polymers, in this case polystyrene, may be investigated from two different points of view. One approach is based on the evolution of volatile products, for example the determination of volatilization rates. The other approach is based on investigations of the liquid phase of the molten polymer. As an extension of former work that was based on methods determining the gasification rates, this work focuses on the liquid phase of the degraded polymer. Two different methods were applied: (1) molecular weight distributions (MWDs), for various pyrolysis conditions, were determined by means of off-line analysis with gel permeation chromatography; and (2) on-line electron spin resonance was used to observe the formation of radicals which contribute to the radical chain mechanism. This study covers low-temperature investigations in which polystyrene degradation was observed without gasification in a temperature range between 200 and 300 °C, and investigations between 300 and 350 °C with low gasification rates. These pyrolysis conditions were selected to separate the initiation reactions and gasification reactions as far as possible. Numerical simulations are performed to calculate MWDs from measured rate coefficients and additionally obtained literature data.