The thermal degradation of polystyrene dissolved in mineral oil was investigated at 240-300°C. Samples were analyzed by gel permeation chromatography to obtain the dependence of polymer molecular weight distributions (MWDs) on reaction time. Experimental data indicated that polystyrene degrades both by random chain scission and specific chain-end scission. The existence of weak links was evident, as indicated by rapid chain scission during an initial time period. The MWDs were analyzed with a continuous-mixture kinetics model to obtain separate rate coefficients for weak-link and strong-link scission. Rate coefficients for the specific chain-end degradation of polystyrene to monomers and oligomers were also obtained. Activation energies of the weak- and strong-link rate coefficients were 2-5 and 7-5 kcal/mol, and chain-end scission activation energies were 0-6 and 0-3 kcal/mol for monomer and oligomer production, respectively. © 1997 Elsevier Science Limited.