Abstract: SnO.sub.2 gas sensors are described incorporating Bi.sub.2 O.sub.3 in an amount less than 35% but sufficient to confer hydrogen sensitivity and selectivity to the sensors described comprises 25% by weight Bi.sub.2 O.sub.3 and incorporates a platinum catalyst.

Abstract: Tin oxide sensors are made by mixing antimony bearing material with tin oxide powder and formation of the sensor by deposition of a slurry of the mixture onto a substrate and drying and sintering the slurry, the antimony bearing material being present in an amount sufficient to render the sensitivity of the sensor to one or more of the gases H.sub.2, CO, or CH.sub.4 , relatively independent of the concentration of oxygen in the range P.sub.O2 10.sup.-1 -1 atm. A further type of a tin oxide gas sensor is disclosed having a resistivity that at a measuring temperature increases with concentration of at least one gas to be measured, the sensor is made by calcining the tin oxide in air at a temperature in excess of 1400.degree. C., or otherwise treating the tin oxide so that it has a state of physical aggregation consistent with being formed in such manner. At a second measuring temperature the resistivity of the sensor to said one gas decreases with increasing gas concentration.