Thermodynamic concepts required for the thermodynamic calculation of the potentials of electrodes for high temperature applications are briefly reviewed. A thermodynamic approach to the calculation of half cell potentials and the standard chemical potential of an electron at high temperatures which are related to the Standard Hydrogen Electrode(SHE) is discussed. As examples, an external Ag/AgCl reference electrode and a YSZ(Ag|O_2) pH sensor for high temperature applications are analyzed by using the thermodynamic approach to derive a high temperature pH measurement equation. The two electrodes are employed to measure high temperature pH and the measured pH was compared with the calculated pH by using a solution chemistry method. Concepts and principles for electrode kinetics are also briefly introduced and a modification to the Tafel equations is suggested. Thermodynamic concepts required for the thermodynamic calculation of the potentials of electrodes for high temperature applications are briefly reviewed. A thermodynamic approach to the calculation of half cell potentials and the standard chemical potential of an electron at high temperatures which are related to the Standard Hydrogen Electrode ( SHE) is discussed. As examples, an external Ag / AgCl reference electrode and a YSZ (Ag | O_2) pH sensor for high temperature applications are analyzed by using the thermodynamic approach to derive a high temperature pH measurement equation. The two electrodes are employed to measure high temperature pH and the measured pH was compared with the calculated pH by using a solution chemistry method. Concepts and principles for electrode kinetics are also briefly introduced and a modification to the Tafel equations is suggested.