An SnO2 based gas sensor composition with Bi2O3 and Sb2O3 is shown to respond selectively to CO gas even in the presence of other gases. X-ray diffraction and laser-Raman studies of the compound confirm the formation of bismuth stannate (Bi2Sn2O7) above 800 degrees C, which seems to act as a molecular sieve. The operating temperature for maximum sensitivity is around 200 degrees C. Temperature programmed desorption studies were conducted to determine the nature of oxygen species adsorbed on the semiconductor surface responsible for the gas sensing.