Abstract: A method of determining the oxygen partial pressure in an atmosphere at a known temperature, in which the magnitude of a critical potential equal and opposite to the free energy of formation of an oxide of a metal such as palladium in contact with the atmosphere is determined to provide a measure of the partial pressure of oxygen. Apparatus for use in the method comprises an electrolytic cell including an electrode of the said metal, an electrolyte such as yttria stabilized zirconia having ionic oxygen mobility and means to apply a potential to the cell to modify the resistance characteristics of the electrode.