Significant electrode polarization leads to the underestimation of apparent oxygen ion transference numbers of mixed ionic-electronic conductors, determined by measurement of either e.m.f. of oxygen concentration cells or Faradaic efficiency. In addition to e.m.f. data under open-circuit conditions, the exact measurement of the transference numbers requires no less than two parameters describing the system such as oxygen permeation flux, bulk resistance, voltage or current under closed-circuit conditions. Selected examples of experimental routes, aimed at taking electrode polarization into account, are considered. An appropriate choice of experimental procedures used in e.m.f. or Faradaic efficiency measurements, in combination with the use of electrodes with high polarization resistance, enables significant accuracy improvement in the determination of very small electronic contributions to the total conductivity. For potentiometric oxygen sensors, the presence of minor electronic conduction was shown to result in great experimental errors in the oxygen partial pressure measurement if the polarization resistance of sensor electrodes is high. Analogously, minor electronic conductivity in solid electrolytes may lead to a large error of the polarization resistance determination by impedance spectroscopy in conditions where the polarization resistance significantly exceeds the bulk resistance of the electrolyte.