During accelerating aging, experimental uncertainty may arise due to variability in the oxidation process, or due to limitations in the technique that is ultimately used to measure oxidation. The purpose of the present interlaboratory study was to quantify the repeatability and reproducibility of standard accelerated aging methods for ultra-high molecular weight polyethylene (UHMWPE). Sections (200 microm thick) were microtomed from the center of an extruded rod of GUR 4150 HP, gamma irradiated in air or nitrogen, and circulated to 12 institutions in the United States and Europe for characterization of oxidation before and after accelerated aging. Specimens were aged for 3 weeks at 80 degrees C in an air circulating oven or for 2 weeks at 70 degrees C in an oxygen bomb (maintained at 503 kPa (5 atm.) of O2) in accordance with the two standard protocols described in ASTM F 2003-00. FTIR spectra were collected from each specimen within 24 h of the start and finish of accelerated aging, and oxidation indices were calculated by normalizing the peak area of the carbonyl region by the reference peak areas at 1370 or 2022 cm(-1). The mean relative interlaboratory uncertainty of the oxidation data was 78.5% after oven aging and 129.1% after bomb aging. The oxidation index measurement technique was not found to be a significant factor in the reproducibility. Comparable relative intrainstitutional uncertainty was observed after oven aging and bomb aging. For both aging methods, institutions successfully discriminated between air-irradiated and control specimens. However, the large interinstitutional variation suggests that absolute performance standards for the oxidation index of UHMWPE after accelerated aging may not be practical at the present time.