The environmental degradation of high-density polyethylene (HDPE) has been studied, in addition to that of HDPE blends, containing various concentrations of ethylene carbon monoxide copolymer. Extruded sheets of each material were exposed to natural Arizona sunlight for times up to 6 months. Exposed samples were then analyzed with respect to molecular weight, density, thermal behavior, mechanical properties, and infrared absorption. Additional samples were exposed to laboratory weathering conditions, evaluated in terms of property changes, melted, reformed, and then reevaluated without further weathering exposure. Results indicate that sunlight exposure causes decreased elongation to break, increased embrittlement, decreased molecular weight, and increased crystallinity. Environmental oxidative degradation is elucidated by the measurement of specific infrared bands, sensitive to the formation of carbonyl and vinyl end groups. As environmental degradation causes reductions of molecular weight, polymer chain mobility increases, leading to a higher degree of crystallinity. This increased crystallinity, along with the decreased molecular weight, accounts for the loss of ductility, indicated by a sharp decrease in ultimate elongation. The presence of carbon monoxide copolymer in the blended samples accelerates the process of environmental degradation, however, the degradation mechanisms appear to be similar to those observed for nonblended HDPE.