Surface-enhanced IR absorption spectroscopy (SEIRAS) was applied to in-situ observation of the adsorption states and the photocatalytic decomposition of acetic acid on Au/TiO2 and Au/Pt/TiO2 films. The adsorption states of acetic acid were determined in both the liquid and gas phase, and molecular adsorption was found to be dominant in the gas phase. Pt deposition on the TiO2 film led to the formation of adsorbed methyl during the photodecomposition of gas-phase acetic acid, indicative of the formation of methyl radical as a reaction intermediate. The addition of gas-phase water to the system led to a significant increase in the coverage of adsorbed methyl as a result of enhancement in the reaction rate as reported before.