The mechanism of methanol oxidation on a Au/TiO 2 catalyst was investigated by combining in situ modulation-excitation diffuse reflectance infrared spectroscopy (MES-DRIFT) with periodic density functional theory (DFT). Adsorbed reaction intermediates and spectators were identified. Methanol dissociatively adsorbs as a methoxy ( CH 3 O ) species on the titania surface, and stepwise oxidizes to formate, carboxyl and CO, carbonates and CO 2 at the Au/ TiO 2 interface. The role of the metal-support interface is clearly observed experimental and theoretically according to a metal–Au-assisted Mars van Krevelen mechanism. Energy barriers were calculated for each elementary step, and the methoxy oxidation was found to be the rate-limiting step in the proposed scenario.