Oxygen-containing chemical species may cause an artificially high level of measured oxidation in the infrared (IR) analysis of clinically retrieved or hip wear simulator tested ultra-high molecular weight polyethylene (UHMWPE) implants. Esterified fatty acids from the synovial fluid or calf serum commonly used in the hip or knee wear simulators are the most likely source of contamination. No such interference is encountered in the IR analysis of shelf-aged UHMWPE components that have never been implanted. When the contamination is observed, an additional IR peak at 1740 cm-1 appears along with the IR peak at 1717 cm-1 commonly assigned to the ketone group. A two-step decontamination procedure, with components first washed ultrasonically in a deionized water bath followed by an ultrasonic washing in heptane, was used in the study to remove possible contamination. This procedure is similar to one reported by James et al. Change in oxidation index before and after the two-step washing varied from 30 to 58% in the articulating region from a depth of 0 to 200 μm. From these results, the effect of contamination is significant and therefore a two-step washing procedure prior to IR analysis to remove water-soluble and organic contaminants is recommended.