Ceramic ZrO2, TiO2and related oxides with suitable O2‐sensitive electrical properties have found important applications in devices for measuring exhaust‐gas O2concentration. For example, such devices are key components in feedback control systems that would maintain the intake air‐to‐fuel ratio near the stoichiometric value where regulated emissions can be minimized. The physical principles underlying the operation of ZrO2based O2‐concentration cells and TiO2‐based resistive devices for the stoichiometric application are described. Finally, a device based on electrochemical O2pumping is discussed which may be useful for A/F control in the fuel‐efficient lean region.