In an attempt to explain the electrochemical effects observed on an original potentiometric gas sensor, the influence of oxygen pressure and temperature on the electrochemical response of the device were largely investigated. The sensor consists of a solid electrolyte associated with two different electrodes, one in platinum the other in gold, located in the same gas mixture. A thermodynamic model is proposed, based first on the competitive adsorption of two oxygen species on the metals and second on the capacitance effects which may occur at the metal solid electrolyte interface. The simulation of the model provides a satisfactory solution if one of the oxygen species is considered as a metastable particle responsible for the electrostatic potential that appears at the metal-solid electrolyte interface. Additional electrostatic experiments confirm the role of oxygen in the potentials of platinum and gold which appear as ideally polarizable electrodes.