The surface species responsible for NO2 gas sensing over indium oxide was studied by operando DRIFTS coupled to a multivariate spectral analysis. It revealed the important roles of surface nitrites on the temperature-dependent gas sensing mechanism and the interaction of such nitrites with surface hydroxyls. A highly hydroxylated surface with high concentration of surface adsorbed H2O is beneficial to enhance the concentration of adsorbed NO2, present as nitrites, thus explaining superior sensing response at lower operating temperatures.