Ultrahigh molecular weight polyethylene (UHMWPE), the current polymer of choice in orthopedic prosthetic devices, is typically sterilized by exposure to Co-60 gamma irradiation prior to packaging for long-term storage. However, the exposure to Co-60 irradiation generates free radicals along the polymer chain that can participate in a series of reactions commencing with the oxidation of the free radicals to form reactive peroxy radicals. This study was undertaken to identify the role of hydroperoxide species in shelf-aged and accelerated aged UHMWPE samples by using a nitric oxide derivatization technique. It is shown that the concentration of hydroperoxides did not change appreciably with shelf aging. However, during accelerated aging the hydroperoxide concentration increased to a plateau and then decreased, suggesting its role as an intermediate in the process. By contrast, the concentrations of carbonyl species continued to increase during shelf aging and accelerated aging. The effects of several packaging materials on the oxidation characteristics were also investigated. A vacuum foil package is shown to be effective in preventing oxidation to a significant extent during accelerated aging. However, accelerated aging after removal from the foil pack resulted in oxidative degradation. Extended vacuum to remove dissolved oxygen and a 5-week room-temperature healing process in the foil pack were shown to be ineffective in reducing oxidative degradation. It also was shown that increased moisture content in the aging environment did not affect the degradation process.