We have determined in situ the phase stability region of a Mn-Zn ferrite spinel Mn0.54Zn0.35Fe2.11O4 from the isothermal variation of total conductivity with oxygen partial pressure (P-O2) at elevated temperatures and its mean cationic transference number via a modified Tubandt-like electrotransport experiment in its entire stability range. At a given temperature, the ionic transference number increases with increasing P-O2 in oxidizing atmospheres and with decreasing P-O2 in reducing atmospheres, leaving a minimum in between. By combining the total conductivity and the transference number, the partial ionic conductivity has been extracted and analyzed in the light of defect structure and mobility of the ferrite spinel. It has also been observed that an electrotransport-induced unmixing occurs especially at interstitial-dominating reducing atmospheres.