The effect of water on the adsorption and photocatalytic decomposition (PCD) of formic acid on TiO2 and Pt/TiO2 was investigated using transient reaction studies, temperature-programmed desorption (TPD), and Fourier transform infrared (FTIR) spectroscopy. Reaction studies indicated that physisorbed water increased the PCD rate of formic acid to a small extent on TiO2 and to a major extent on Pt/TiO2, but the presence of only chemisorbed water did not. FTIR spectroscopy and TPD studies indicate that the main effect of the addition of water to TiO2 that had adsorbed formic acid was the displacement of formic acid. However. FTIR spectroscopy indicated that the addition of water caused a change in the adsorbed structure of formate that may be associated with the higher reactivity. 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.