Free-radical-depth profile of a large cube (side, 25.4 mm) of UHMWPE was determined following gamma irradiation (25 kGy, 6 0 Co) in air and subsequent aging for 120 days in ringer solution, bovine serum or ethyl alcohol at 37 o C, or in air at room temperature (RT). The initial electron spin resonance (ESR) data showed the presence of predominantly allyl radicals in the core specimens (>6 mm from the surface), alkoxy (PO ) or peroxy (PO 2 ) in the surface (1-3 mm from the surface), but none or very weak radicals in the sub-surface region (3-5 mm from the surface). Ten months later, the same samples, including those in the sub-surface region, produced identical surface radicals. The results of this study led to the conclusion that the transient non-radical species observed, for the first time by ESR technique, could be the well-known hydroperoxy (POOH) group.