H 2S gas interaction mechanisms of sputtered SnO 2 and SnO 2-CuO bilayer sensors with a varying distribution of the Cu catalyst on SnO 2 are studied using Pt interdigital electrodes within the sensing film. Sensitivity to H 2S gas is investigated in the range 20-1200 ppm. Changes induced on the surface, the SnO 2-CuO interface, and the internal bulk region of the sensing SnO 2 film upon exposure to H 2S have been analyzed to explain the increasing sensitivity of three different sensors SnO 2, SnO 2-CuO, and SnO 2 with CuO islands. SnO 2 film covered with 0.6 mm diameter ultrathin (∼10 nm) CuO dots is found to exhibit a high sensitivity of 7.3×10 3 at a low operating temperature of 150°C. A response speed of 14 s for 20 ppm of H 2S, and a fast recovery time of 118 s in flowing air have been measured. The presence of ultrathin CuO dotted islands allow effective removal of adsorbed oxygen from the uncovered SnO 2 surface due to spillover of hydrogen dissociated from the H 2S-CuO interaction, and the spillover mechanism is sensed through the observed fast response characteristics, and the high sensitivity of the SnO 2-CuO-dot sensor.