Abstract: A sensor responds sensitively to low concentrations of carbon dioxide in an atmosphere of air. Carbonates formed at the surface of preferred oxides can be decomposed by raising the temperature, thus reforming the original oxide. A CO2 sensor operating according to this principle may be cycled between the temperature at which the surface carbonate is decomposed and the temperature at which the surface carbonate is formed from atmospheric carbon dioxide. The present invention relates to sensors and more particularly to sensors suitable for use in gaseous mixtures containing carbon dioxide.