Comparison of photooxidation rates of formic acid and formate on TiO2 as well as the effect of adsorbed water on formate and formic acid photodecomposition rates have been investigated by Fourier transformed infrared spectroscopy. Adsorbed formic acid and formate are all photooxidized to CO2 in O-2. Formic acid on TiO2 shows a photoreaction rate that is roughly 53 times that of formate groups for a same surface concentration. The presence of H2O can increase formate and formic acid photooxidation rates by a factor close to 2. In addition, photochemistry of adsorbed ethanol and ethoxy is also compared. It is found that ethanol is important for the formation of acetaldehyde, while ethoxy groups are photooxidized to adsorbed acetate, formate, and water. Possible reasons for these differences are discussed.