Zirconia powders were prepared by heating at 470 degrees C hydrothermal precursors obtained by precipitation at increasing pH (from 2 to 12). The phase composition of the calcined products varied from pure monoclinic (at the acid extreme) to pure tetragonal (neutral-alkaline range). For the mixed phase samples, the relative enrichment in the two polymorphs was estimated by interpretation of X-ray diffractograms. Electrochemical determinations of the double layer reactivity of the pure and mixed phase oxides were performed and combined with characterisations of the surface state by X-ray photoelectron spectroscopy (XPS). Interpretations of the distinctive electrochemical features shown by the different polymorphs are proposed.