A carbon monoxide gas sensor based on semiconducting oxide SnO2 has been developed. The addition of small amounts of transition metal oxide, MoO3, not only makes the elements highly selective to CO but significantly enhances the sensitivity of the sensor element in terms of the magnitude of the signal as well as the response time. The sensor element has been tested for cross selectivity to other reducing gases and confirmed unambiguously to be specifically sensitive to carbon monoxide gas. The sensor elements have been tested under different operating conditions. Different analytical techniques were used to establish the mechanism of sensitivity specifically to CO. © 1995 American Institute of Physics.