Use of ceramics for fabrication of oxygen sensors is reported. The phase stability, electrical conductivity, and thermoelectric power of these solid solutions with have been investigated in the range 700°–1100°C and for oxygen partial pressures in the range 1–10−20 atm. The nonstoichiometric defects appear to be singly and doubly ionized cobalt vacancies and give rise to a p‐type conductivity. These materials have properties, including stability over a wide range of oxygen pressures and temperatures, that make the oxygen sensors particularly suited for monitoring and control of combustion processes. Some results on sensor performance in an internal combustion engine are presented.