ABSTRACTThe semiconductor properties of a praseodymium-cerium oxide solid solution with composition Pr_0.15Ce_0.85O_2-x (PCO) were investigated by d.c. current-voltage and bias-dependent impedance measurements in aqueous solution. The solution data were compared with impedance values of dry cells in air. A Mott-Schottky analysis of the PCO-solution interface capacitance showed p-type semi conductivity, a flat-band potential E_fb = (2.0 ± 0.1) V/NHE and an ionized acceptor density NA = 3 10 cm_-3. Using these data, an electron hole mobility μ_h ∼ 10_-5 cm₂ V_-1 s_-1 was calculated pointing to a small polaron conduction mechanism with a hopping energy (E_h = 0.4 eV).