Decarboxylation is an important process in the photocatalytic degradation of organic pollutants. In this study, the adsorption and photocatalytic decarboxylation of acetic acid and trichloroacetic acid were investigated by in-situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), which can distinguish carboxylate groups with different adsorption modes. We found that water molecules promote photocatalytic decarboxylation reaction and the promotional effect of water is attributed to the changed adsorption mode of the carboxylate group by the co-adsorbed water molecules. The IR study shows that degradation of the monodentate-coordinated acetic acids is much faster than that of the bidentate-coordinated one. The kinetic isotope effect studies and analysis of the intermediate products indicates that the degradation of acetic acids originates from the direct oxidation by holes even in the presence of water, rather than from the reaction of OH radicals as generally believed. DFT-based molecular dynamics calculations revealed that the formation of the monodentate-coordinated carboxylate group in the presence of water is attributed to competition between the O atom of the carboxylate group and water for the surface Ti sites. Because of its higher electron density, the monodentate carboxylate group is easier to be directly oxidized by holes than the bidentate group.