Gas sensors have been used and available for many years. While there are many different types of gas sensors and detecting materials, metal oxides are commonly used for the detection of various gases, such as carbon monoxide (CO), methane (CH4), and other hydrocarbons. Such sensors are used in environmental, commercial HVAC, home, and industrial applications. One of these metal oxides, tin oxide, is very stable in air at temperatures > 500 degrees C, and is readily processed using ceramic or thin-film techniques. The thin films on silicon can be doped with inpurities using standard diffusion or spin-on methods, which influence the sensitivity, selectivity, and stability of the sensors. Thin- film tin oxide on silicon CO gas sensors have been characterized. The structure, chemistry, and response results are reported.