The dynamics of polymer decomposition by random-chain scission can depend on the macromolecular chain length (or molecular weight, MW). We have investigated how the MW dependence of the rate coefficient can affect the degradation of polystyrene (2 g/L) dissolved in mineral oil at 275 °C. The evolution in time of polymer MW distributions (MWDs) was monitored by gel permeation chromatography of polymer samples taken from the reactor. The degradation (based on first-order continuous-distribution kinetics) of monodisperse polystyrene of initial MWs ranging from 12 000 to 0.93 million showed a substantial effect of MW. The experimental data indicated that polystyrene degraded mainly by random-chain scission and that chain-end scission was much less significant. The rapid reaction at initial times demonstrated the existence of weak links in the polymer chain. Equations are proposed for obtaining the degradation rate by examining the time evolution of the MWD as measured by the change in MW moments. The moments were analyzed with the model to obtain the rate coefficient as a second-degree polynomial function of the MW. The results demonstrate how the change in MWD with time can be predicted by taking into account the change in degradation rate with MWD. © 1997 American Chemical Society.