Pyrolysis of polystyrene (PS) plastic waste was investigated in this experimental work harnessing a pressured autoclave surrounded by a furnace. PS was thermally degraded with various organic compounds, such as phenol, quinone, naphthalene, and diphenylamine. Experiments varied the temperature between 350 and 450degreesC. The main products of PS waste pyrolysis were mainly styrene monomer, ethyl benzene, toluene, and methyl styrene. The product spectrum can be described as a function of pyrolysis temperature and used organic compounds. The yield styrene of liquid products at various temperatures and at 400degreesC and 60 min with organic compounds were about from 60 to 74%. The optimum pyrolysis temperature to maximize styrene monomer yield (about 60%) was 400degreesC, and the maximum styrene yield was obtained with naphthalene as 74% in this study. The amount of styrene was found to increase in the following order: diphenylamine < thermal < phenol < quinone < naphthalene.