The coadsorption of water and preadsorbed oxygen on Ru͕0001͒ was studied by synchrotron-based highresolution x-ray photoelectron spectroscopy. A dramatic change was observed in the interaction of water with oxygen between low and high oxygen precoverages. Low oxygen coverages below 0.18 ML induce partial dissociation, which leads to an adsorbed layer of H 2 O and OH. Around half the oxygen atoms take part in this reaction. All OH recombines upon heating to 200 K and desorbs together with H 2 O. Oxygen coverages between 0.20 and 0.50 ML inhibit dissociation, instead a highly stable intact water species is observed, which desorbs at 220 K. This species is significantly more stable than intact water on the clean surface. The stabilization is most likely due to the formation of hydrogen bonds with neighboring oxygen atoms. For intermediate oxygen coverages around 0.18 ML, the dissociation behavior depends on the preparation conditions, which points toward possible mechanisms and pathways for partial dissociation of water on Ru͕0001͖.