The potential–pH response of an electrolytic manganese dioxide is investigated by means of a cavity microelectrode (CME). The potential–pH curves show a complex evolution that could be explained by the disporportionation of MnOOH species, leading to the formation of Mn2+ ions on the MnO2 surface. Such a behaviour is not suited for pH sensor application. However when the tip of the electrode is coated by a Nafion membrane, the potential–pH evolution shows a unique slope close to −60 mV pH−1. In addition, the sensor exhibits short time responses to pH variations, a good selectivity, and it can be easily renewed compared to classical sensors.