The effects of water on the carboxylic acid ketonization reaction over solid Lewis-acid catalysts were examined by nuclear magnetic resonance (NMR) spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), temperature-programmed desorption (TPD), and kinetic measurements. Acetic acid and propanoic acid were used as model compounds, and P25 TiO2 was used as a model catalyst to represent the anatase TiO2 since the rutile phase only contributes to <2.5% of the overall ketonization activity of P25 TiO2. The kinetic measurement showed that introducing H2O vapor in gaseous feed decreases the ketonization reaction rate by increasing the intrinsic activation barrier of gas-phase acetic acid on anatase TiO2. Quantitative TPD of acetic acid indicated that H2O does not compete with acetic acid for Lewis sites. Instead, as indicated by combined approaches of NMR and DRIFTS, H2O associates with the adsorbed acetate or acetic acid intermediates on the catalyst surface and alters their reactivities for the ketonization reaction. There are multiple species present on the anatase TiO2 surface upon carboxylic acid adsorption, including molecular carboxylic acid, monodentate carboxylate, and chelating/bridging bidentate carboxylates. The presence of H2O vapor increases the coverage of the less reactive bridging bidentate carboxylate associated with adsorbed H2O, leading to lower ketonization activity on hydrated anatase TiO2. Surface hydroxyl groups, which are consumed by interaction with carboxylic acid upon the formation of surface acetate species, do not impact the ketonization reaction.