NiO electrodes, prepared by pressing NiO and alpha-zirconium phosphate powders (1:1 or 1:2 in weight) were anodically doped with Ni(III). The use of these electrodes as solid references of gas sensors based on solid state protonic conductors has been investigated. A very low drift of potential (less than or equal to 0.1 mV per day) and an exchange current density of the order of about 2 mu A/cm(2) were found. Because of the good stability of its potential and the relatively high exchange current density, this kind of electrode can be suitably used as both reference and as counter-reference electrode in potentiometric and amperometric sensors, respectively.