An investigation has been made of the magnitude and the source of degradation of polystyrene in model extrusion experiments. A preformed solid plug of a narrow-distribution, high molecular weight polystyrene (M̄w = 2 × 106) was used for each experiment. The extrusions were performed with an Instron capillary rheometer at constant temperatures from 160° to 220°C. The molecular weight changes occurring in the capillary reservoir during the pass of the capillary and in the hot extrudate were evaluated. Molecular weights and distributions were obtained by gel permeation chromatography. A significant degradation was observed in the capillary reservoir. The degree of degradation was independent of polymer flow. The polystyrene near the walls reacted more extensively than that closer to the axis, but a constant degradation was observed along the axis. The molecular weight profile across the capillary reservoir was attributed to a selective thermo-oxidative degradation (160° and 190°C). Changes in molecular weight due to mechanical degradation in the capillary were observed only at 160°C. This reaction was referred to as a mechano-oxidative process, as no degradation could be detected in the absence of oxygen. At 160°C, selective migration of macromolecules (fractionation) during extrusion did not occur to any significant extent, nor could any thermo-oxidative degradation of the extrudate be detected. When a small amount of antioxidant (Ionol) was added to the polystyrene, no selective degradation did occur at the walls of the capillary reservoir during extrusion at 160° and 190°C.