Since the 1960s ultrahigh molecular weight polyethylene has been a primary bearing material in orthopedic prostheses. Most polyethylene components have been sterilized by exposure to β₯ radiation, but in the presence of oxygen this promotes degradation and correlates with component failure in the body. To better understand the mechanism of degradation, we measured oxidation profiles for components shelf aged 5.8 and 10.9 years. Microtomed sections from each component were analyzed for ketones, which are the primary oxidation products, by FTIR spectroscopy. Maximum ketone concentrations in both cases were found several millimeters below the surface. We propose a model to explain the profiles. Alkyl radicals, which are uniformly generated in the polyethylene during irradiation, react with peroxyl radicals, which are produced by alkyl radi-cals with oxygen to form ketones. At the component surface ketone levels are low because high oxygen concentrations deplete alkyl radicals, creating an excess of peroxyl radicals. In the bulk material ketones are low because low oxygen concentrations limit the formation of peroxyl radicals. However, just below the surface the concentrations of alkyl and peroxyl radicals balance, promoting ketone production. A computer simulation that accounts for the coupling of these reactions with the diffusion of oxygen shows a good agreement with the measured profiles.