The formation and reactivity of hydroxyl species originating from coadsorption of water and oxygen on Ni(110) single-crystal surfaces have been studied by using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The resulting surface population of hydroxyl intermediates at a given water-oxygen coverage combination was found to be temperature-dependent. This was demonstrated by the differences in hydroxyl coverages determined by TPD and XPS: while the TPD data were determined to mostly reflect the maximum coverages that can be reached for a given set of gas exposures at low temperatures, the XPS results measure the OH coverages formed at the temperature of dosing. Our results indicate that, besides the stoichiometric and reversible H2O(ads) + O(ads) = 2 OH(ads) step, a second water-decomposition reaction on the oxygen-precovered surface deposits additional hydroxyl adsorbates. Depletion of surface oxygen can be induced by thermal reaction with coadsorbed ammonia as well, a result that provides direct evidence for the OH(ads) disproportionation reaction.