Measurements with dilute gaseous mixtures of hydrocarbons and oxygen in a nitrogen carrier gas have shown that the workfunction of a Pt film in interaction with the gas phase changes abruptly and reversibly by approximately 0.7 eV at elevated temperature as the concentrations of the chemically active species are varied through their stoichiometric ratio. This effect provides a new mechanism for sensing the stoichiometric air-to-fuel ratio (A/F) in exhaust or flue gas applications. The phenomenon of the thermionic emission of alkali metal ions in which the rate of emission increases strongly with the workfunction of the emitting surface offers an electrical means for detecting the workfunction change as well as a simple structure for realizing a practical device.