SnO2 thin films were prepared by the rf reactive sputtering method from a target of the same compound in order to be used as gas sensors. Dynamic response characteristics of the electrical conductance to different CO concentrations in dry air are presented for pure films and for films activated with a suitable amount of metallic Pt deposited by sputtering onto a SnO2 surface. The sensitivity, the response, and recovery times as a function of temperature were analyzed. In particular, the Pt-SnO2 films show relatively high sensitivity to CO gas and good stability at an operating temperature of about 170 degrees C.