The chemistry of metal oxide surfaces has long been thought to be dominated by reactions involving defects(1,2). These are minority sites such as oxygen vacancies. Thus far, it has proved difficult to obtain direct experimental evidence to support this idea, although some progress has been made(3-5). Here, we use the scanning tunnelling microscope (STM) to image the reaction of water molecules with bridging-oxygen vacancies on a model oxide surface, rutile TiO(2)(110). In a form of single-molecule chemistry, individual oxygen vacancies are observed being transformed into OH species as a water molecule dissociates in the vacancy. We use the STM tip to selectively desorb individual H atoms, whilst leaving the vacancies intact. This allows us to distinguish between vacancies and OH, which have a similar appearance in STM. In a very clear way, these results validate the view that defects can play a key role in oxide surface reactions.