Photocatalytic oxidation (PCO) of formic acid on TiO2 produces CO2 without forming any long-lived intermediates. Formic acid PCO is not limited by surface diffusion of either formic acid or active oxygen species. The formic acid oxidation rate increases significantly with temperature up to 373 K, which was the highest temperature used in this study. At least two types of active sites exist for PCO of formic acid on TiO2 and their activities differ by a factor of at least 20. The more-active sites comprise less than one-fourth of a room-temperature formic acid monolayer. Water, which is produced during PCO, redistributes adsorbed formic acid by displacement. Water readily displaces approximately one-third of a formic acid monolayer but it does not effectively compete for the remaining formic acid adsorption sites.