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Metal-oxide semiconductors are of great interest for gas-sensing applications. We provide new insights into the mode of operation of ceria-based gas sensors during ethanol gas sensing using combined operando Raman–gas-phase FTIR spectroscopy. Visible Raman spectroscopy is employed to monitor the presence of oxygen vacancies in ceria via F₂g mode softening, while simultaneously recorded FTIR spectra capture the gas-phase composition. Such an experimental approach allowing the direct observation of oxygen vacancies in metal-oxide gas sensors has not been reported in the literature. By systematically varying the gas atmosphere and temperature, we can relate the sensor response to the spectroscopic signals, enabling us to obtain new fundamental insight into the functioning of metal-oxide semiconductor gas sensors, as well as their differences from heterogeneous catalysts.