Composite structures can be made from stabilized zirconia ceramics sintered against a semiconducting oxide so that potential measurements reveal the progress of gas reactions on the surface of the semiconductor. Effectively the zirconia component is used as a high-impedance probe of the surface states on the semiconductor. The usual limitation of the potentiometric oxygen sensor, that the test atmosphere must be separated from a reference atmosphere, is avoided here by operating at temperatures below that at which oxygen species on the semiconductor surface are in equilibrium with the gas atmosphere. Cells of the sort platinum/yttria-stabilized zirconia/semiconducting oxide offer substantial mixed potentials in air over most of the temperature range between room temperature and around 400 °C. In addition, the cell potential shows large responses to the introduction of reducing gas (hydrogen, carbon monoxide, ethene etc.) over just the same temperature range as that in which resistance-modulating semiconductor sensors respond.