Dynamic-mechanical measurements, i.r. spectroscopy and oxygen absorption techniques have been used to study structural changes in high impact polystyrene during its photo-oxidation under artificial weathering conditions. It was found, using a viscoelastometric technique, that the damping peak at −80° (corresponding to the β-transition temperature of polybutadiene) disappeared completely after irradiation for 14 hr; this was accompanied by a sharp increase in the complex modulus at 20°. A parallel change was observed in the rate of formation of carbonyl and hydroxyl groups as measured by i.r. spectroscopy. The results indicate that the polybutadiene part of the resin is selectively attacked by u.v. light during the initial stages of the process. Similar results have been reported for ABS [1].