The MnO2,-montmorillonite composite mass has been investigated as an all-solid internal reference for pH sensors. Owing to its protonic conducting and cationic exchange features, protonated montmorillonite ensures a reversible cationic exchange (H+/Na+) between the internal active mass and the glass membrane. The effect of the mixture composition and that of relative humidity inside the electrode have been studied. The electrode consisting of MnO2 (75%), H-montmorillonite (20%) and acetylene black (5%) displays proper behavior at relative humidities greater than 0.8. The performances of such a sensor are comparable to those of a conventional glass electrode.