Waste plastics are an environmental problem because of recycling limitations and their resistance to natural decomposition. This research investigates the thermal degradation (pyrolysis) of waste polystyrene in heavy oil as a tertiary recycling method. Kinetic parameters for pyrolysis were obtained, and distillate products from the reaction were characterized by gas chromatography/mass spectroscopy. Four types of heavy oil were used to find which would be most suitable in this process. Complete pyrolysis of polystyrene occurred at temperatures lower than those reported in the literature.