The reaction of water with stoichiometric and O-defective UO2 thin film surfaces is studied by high-resolution photoelectron spectroscopy using synchrotron X-rays radiation. The decomposition of D2O molecules and the oxidative healing of defects on the reduced surfaces was observed and quantified. D2O adsorption on the stoichiometric UO2 surface at 300 K showed small amounts of OD species (ca. 532 eV) probably formed on trace amounts of surface defects, while at 95 K D2O ice (533.5 eV) was the main surface species. On the contrary, a large signal of OD species was seen on the 300 K-Ar+-sputtered (reduced) surface, UO2-x. This was concomitant with a rapid healing of surface defects as monitored by their U4f signal. Quantitative analysis of the OD signal with increasing temperature showed their disappearance by 550 K. The disappearance of these species while hydrogen molecules are still desorbing from the surface as monitored by TPD [S.D. Senanayake, H. Idriss, Surf. Sci. 563 (1-3) (2004) 135; S.D. Senanayake, R. Rousseau, D. Colegrave, H. Idriss, J. Nucl. Mater. 342 (2005) 179] is shedding light on the re-combinative desorption mechanism from dissociatively adsorbed water molecules on the surfaces of this defective metal oxide. (c) 2006 Elsevier B.V. All rights reserved.