Isothermal pyrolysis of PS was investigated in the relatively low temperature range of 370 to 400℃, using a stirred tank of batch-type with respect to the liquid phase ; the gas phase was operated in an open system. To maintain an isothermal condition, the stirring speed was 80rpm. At the initial stage of pyrolysis at each temperature, the first-order kinetic law was found to be operative. The activation energy was evaluated to be 224kJ/mol, a value close to the previously reported one. The main liquid products were single and double aromatic ring species : styrene, α-methylstyrene, toluene and 1,3-diphenylpropane, 1,3-diphenylbutene. The yield of styrene was much larger than any other product and amounted to ca. 70% by weight. The present reactor, operated in an open system with respect to the gas phase, tends to suppress the scission of volatile diphenyl radicals because of short residence times of these radicals.