Sn0₂:Cu0 thin films have been identified earlier as highly sensitive and selective H₂S gas sensors. Usually the sensors are characterized by measuring dc resistance. In the present study ac impedance spectroscopy has been used to investigate the effect of Cupric oxide addition on H₂S sensing properties of polycrystalline Sn0₂ based thin film sensors. For this two types of thin films -pure Sn0₂ and Sn0₂:(0.5wt%)CuO, were studied. It has been observed that the total impedance of the films has two contributions: one due to grains (intragranular) and other from grain boundaries (intergranular). On exposure to H₂S gas, it.was seen that for Sn0₂ films, the intergranular resistance drops by a factor of 6 while for Sn0₂:Cu0 films this reduction is by a factor of IO⁷. These results support earlier studies, that high sensitivity in SnO₂CuO films is due to destruction of p-n junctions formed at grain boundaries.