Gas sensors have been fabricated by a mixture of SnO2, CuO powder, and tetraethylorthosilicate. Their response to H2S gas has been investigated. A five ppm H2S gas concentration changes their resistivity to a value lower than that in air, even in the presence of other gases. By protecting the sensor surface with organic material, sensitivity and selectivity for H2S gas has been achieved. The observed change in conductivity of these films upon exposure to H2S gas in air has been explained on the basis of band theory of solids supported by our x-ray diffraction data and I-V characteristics.