The effect of water on the adsorption of formic acid on the anatase (1 0 1) surface of TiO2 was investigated with density functional theory. These calculations showed that water co-adsorbed with formic acid promotes O-H bond dissociation to produce formate ions, but that the details of this chemistry depend on the surface coverage of formic acid. For 1/4 monolayer (ML) formic acid coverage, O-H dissociation required only a 1:1 ratio of water to formic acid. At 1 ML formic acid coverage, only 1/4 ML formic acid dissociated unless significant constraints were placed on water adsorption geometries. The density functional theory calculations also indicated that water converts bidentate bridging formate to monodentate formate. Comparisons with experimental data reveal that these transformations can have an important influence on elementary steps in the photocatalytic decomposition of formic acid on TiO2 and Pt/TiO2. (C) 2010 Elsevier Inc. All rights reserved.