The behavior of adsorbed water on oxides is of fundamental interest in many areas. Despite considerable attention received recently, our understanding of water chemistry is still short of needs and expectations, particularly on the topic of the coadsorption of water and other species. In this study we carry out density functional theory calculations to investigate the coadsorption of water and oxygen on the TiO2(110) surface. We show that oxygen exerts profound influences on the water adsorption, altering the mechanism of water dissociation. On the one hand, the possible dissociation route along [-110] is prohibited due to the weakening of the H bond between water and the lattice bridging oxygen in the presence of the coadsorbed oxygen, and on the other hand the coadsorbed oxygen induces dissociation along [001]. These results lead to a consistent interpretation of experiments. Furthermore, several possible final states and the related formation mechanisms are discussed in detail. (C) 2004 American Institute of Physics.