The effect of a tensile stress on the rate of photo-oxidation of polystyrene was investigated. Molecular-weight measurements showed that tensile stress accelerates molecular scission in injection-moulded bars exposed to ultraviolet irradiation. Changes in the residual-stress distribution were observed during the exposure but they were not sufficient to reverse the sense of the residual stresses, and the residual stress near the surface remained compressive. Depth profiling indicated that the degradation process was oxygen-diffusion limited and an approximate analysis of the kinetics gave an activation energy for degradation of 4.2 kJ m-2, which is also consistent with a diffusion process.