The tungsten/tungsten oxide electrode was investigated as a pH sensor over a wide range of pH at temperatures from 200 to 300-degrees-C and was compared with the yttria-stabilized zirconia membrane electrode. The tungsten/tungsten oxide electrode shows a Nernstian pH response within the pH range of 2 to 11 and exhibits stable and reproducible potentials. In solutions with pH values lower than 2 were observed deviations from Nernstian behavior. The potential of the electrode is not significantly affected by the redox properties of the system, as established by the presence of oxygen or hydrogen in the solution.