Tribochemical reaction of DLC coating in water was investigated by using a stable isotopic tracer, O-18 labeled water (H-2 O-18), to carry out the friction test of DLC coating and 440C ball pair, and using ToF-SIMS to analyze the worn surfaces. The result showed that DLC coating tribochemically reacted with water to form hydrophilic hydroxyl and carboxyl groups on surface, and suggested that the formed hydroxyl mainly combined with the secondary or tertiary carbons on the surface. The surface layer on the counter ball mainly consisted of C from the coating, Cr, Fe from the ball and O-18 from water, and was rich in (OH)-O-18. It is thought that the hydrophilic groups formed at the interfaces play an important role in low friction and wear behaviors of DLC coating and the counter part in a water environment. Comparing with that obtained from the test in D2O, the result also suggests that hydrogen/deuterium exchange is easy to occur between the products containing OD on the mated ball and some adsorbates in an ambient air environment. (c) 2007 Elsevier B. V. All rights reserved.