Combination of impedance spectroscopy and open-cell voltage measurements has been used to determine the transport properties of mixed-conducting electrolytes. It is clearly demonstrated experimentally that the ratio of open-cell voltage to the applied Nernst potential can be quite different from the average ionic transference number; the effect of interfacial polarization resistance on determination of transference numbers of a mixed-conducting electrolyte must be carefully analyzed, particularly for a cell based on a thin electrolyte, tested at low temperatures, or exposed to an atmosphere containing low activities of electroactive species.