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Diffuse reflectance infrared Fourier transform (FT-IR) spectroscopy (DRIFTS) was used in combination with resistance measurements to study the mechanism of Au/SnO₂ during ethanol gas sensing and to elucidate the influence of gold on the sensor response. Time-resolved DRIFT spectra during ethanol gas sensing reveal significant differences between Au/SnO₂ and bare SnO₂ regarding the amount of C–H-containing adsorbates, which are less abundant on Au/SnO₂ because of their consumption by the adsorbed oxygen species. Modulation excitation DRIFT spectroscopy (ME-DRIFTS) was applied to Au/SnO₂ in comparison to bare SnO₂, enabling a distinction of the temporal behavior of different C–H-containing surface adsorbates such as acetate and formate. ME-DRIFTS reveals the presence of a new surface species at 2030–2060 cm⁻¹, not detected for unloaded SnO₂ and associated with CO adsorbed on negatively charged gold particles. X-ray photoelectron spectroscopy (XPS) and ultraviolet/visible (UV/vis) spectra confirm the presence of metallic gold, which makes an influence on the electronic properties of the SnO₂ sensor material unlikely. Based on our spectroscopic findings, we postulate a detailed ethanol gas-sensing mechanism and attribute the increase in the sensor response to an oxygen spillover from gold to the surface of tin oxide.