Through an interplay between scanning tunnelling microscopy experiments and density functional theory calculations, the active surface site responsible for the dissociation of water molecules adsorbed on rutile TiO2 (110) was determined unambiguously. The O vacancies in the surface layer were shown to dissociate H2O through the transfer of one proton to a nearby O atom, forming 2 hydroxyl groups for every vacancy. The amount of water dissociation was limited by the density of O vacancies present on the clean surface exclusively. The dissociation process sets in as soon as molecular water was able to diffuse to the active site.Oxygen Vacancies as Active Sites for Water Dissociation on Rutile TiO2(110). R.Schaub, P.Thostrup, N.Lopez, E.Lægsgaard, I.Stensgaard, J.K.Nørskov, F.Besenbacher: Physical Review Letters, 2001, 87[26], 266104 (4pp)