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A number of sensing systems based on indium oxide doped with various metal oxides (In2O3 · WO3, In2O3 · ZnO, In2O3 · RuO2, In2O3 · Gd2O3, and In2O3 · Sm2O3) in amounts of no more than 3–5 mol % and also Au · In2O3 films were studied as sensors for detecting NO2 in air. The working temperature of sensors was 250°C (except for In2O3 · RuO2, for which T = 150–190°C). In2O3 · WO3-based sensors reach a high sensitivity especially at a concentration of NO2 in air higher than 10 ppm (the relative sensor conductivity changes by 2.5 orders of magnitude). However, a shortcoming of this system is an increased response time (7–9 min) as compared to the other studied systems, for which the response time does not exceed 15–20 s. In2O3 · Gd2O3 and In2O3 · Sm2O3 films exhibit the best sensing properties in sensitivity, selectivity, and stability. Various NO2 species adsorbed on the surface of dispersed indium oxide were detected by Fourier-transform IR spectroscopy. The mechanism of changing the conductivity of In2O3 · Gd2O3 films upon detecting NO2 in air is discussed.