A bubbling reactor for pyrolysis of a polystyrene melt stirred by bubbles of flowing nitrogen gas at atmospheric pressure permits uniform-temperature distribution. Sweep-gas experiments at temperatures 340-370 ℃ allowed pyrolysis products to be collected separately as reactor residue(solidified polystyrene melt), condensed vapor, and uncondensed gas products. Molecular-weight distributions (MWDs) were determined by gel permeation chromatography that indicated random and chain scission. The mathematical model accounts for the mass transfer of vaporized products from the polymer melt to gas bubbles. The driving force for mass transfer is the interphase difference of MWDs based on equilibrium at the vapor-liquid interface. The activation energy and pre-exponential of chain scission were determined to be 49 kcal/mol and 8.94×10(13) s(-1), respectively.