As a part of our research on new gas sensors, an original potentiometric gas sensor was developed. This gas sensor is made of a solid electrolyte (β alumina) associated with two different metallic electrodes (gold and platinum) located in the same gas mixture. Tests performed under oxygen pressure show that the potential difference read between the electrodes depends on the experimental conditions, oxygen pressure, and temperature, but also on the electrodes surface area ratio. Then, we studied the electrical responses of devices composed of electrodes of the same nature (gold or platinum) but of different size. These electrode size influences are analyzed in terms of different capacitive effects occurring at the electrode-solid electrolyte interface. If we consider that the adsorption of oxygen species is responsible for the electrostatic potential, the proposed model is able to account for the experimental results. It is then possible to explain and to better understand the importance of the electrodes on the electrical response of such a device.