Ultra-high molecular weight polyethylene (UHMWPE) was irradiated by electron beams to a dose of 100 kGy at a dose rate of 20 kGy min(-1) and the near-surface region of the irradiated polymer was characterized by means of attenuated total reflection Fourier-transform infra-red (ATR FTi.r.) spectroscopy and micro-hardness measurement. The results obtained strongly suggest that irradiation in air introduced hydroperoxide groups into the polymer without formation of any other oxygen-containing groups. The generated hydroperoxides could be decomposed further by subsequent heat treatment of the irradiated polymer, resulting in crosslinking of UHMWPE chains in the near-surface region of the material. As a result of this surface modification, the surface hardness of UHMWPE substantially increased.