The sensitivity of a semiconductor sensor using SnO₂ to H₂S in air could be enormously promoted by loading SnO₂ with a small amount of CuO. However, response kinetics began to deteriorate rather sharply as the H₂S concentration decreased below a certain limit, which depended on CuO loadings. XPS measurements on a series of CuO-SnO₂ samples calcined in air revealed that the binding energies (BEs) for O1s and Sn3d_5/2 levels shifted downward from those of pure SnO₂, while Cu2p_3/2 level showed an upward shift from that of pure CuO. With increasing CuO loading, the shifts in BE increased or decreased linearly for O1s and Sn3d_5/2 levels or Cu2p_3/2 level, respectively, indicating continuous shifts in Fermi levels of SnO₂ and CuO. For a fixed CuO loading, the magnitudes of the BE shifts were dependent on the methods of CuO loading, reflecting differences in the dispersion of CuO particles on SnO₂ grains. These phenomena were well consistent with the formation of p-n contacts between the finely dispersed CuO (p) and the underlying SnO₂ (n) grains. The sensitivity to H₂S was shown to be well correlated with the magnitudes of the BE shifts, indicating that the formation of the p-n contacts in air and the rupture of them upon exposure to H₂S are the origin of the high H₂S sensitivity.