The purpose of this investigation was to characterize the consolidation structure of several types of commercially available, medical-grade ultra-high molecular weight polyethylene (UHMWPE) before and after accelerated oxygen aging. The observations were correlated to the following: (1) UHMWPE resin molecular weight, 2 to 4 versus 4 to 6 million; (2) presence versus absence of calcium stearate; (3) two consolidation methods, ram extrusion versus sheet molding; (4) gamma radiation (2.5 to 3.0 Mrads) versus as-consoli- dated; and (5) consolidation process parameters (manufacturer controlled, “A” versus “B”). Accelerated aging was performed at 70°C, 5 atm pressure, and pure oxygen for 20 days. Optical microscopy was performed on 90-μm-thick sections, with FTIR analysis performed on selected samples. The results of this investigation showed: (1) calcium stearate slightly decreased resistance to oxidation; (2) for a specific resin and manufacturer, compression-molded sheet was more resistant to oxidation than ram-extruded rod; (3) oxidation was not observed for some gamma sterilized UHMWPE while others were significantly oxidized; and last and most important, (4) the resistance to oxidation of a specific UHMWPE resin is most dependent on optimal process parameters (time, temperature, pressure, and cooling, dependent on manufacturer) during resin consolidation, regardless of method. For this study, the most degradation resistant UHMWPE was optimally processed compression-molded GUR 1020 sheet.