The change of the chemical potential of ionic and electronic defects in the phase stability range of a compound has been discussed followed by the correlation of the above parameters with the properties of the phase. A method for the estimation of the maximum change in the Fermi level in the phase stability range has been suggested employing the dependence of the electrical conductivity or the deviation from stoichiometry on the oxidant pressure, or at least, the values of exponents in the function describing the above relation. The analysis of the changes of the chemical potentials of ionic and electronic defects in the oxide phase stability ranges and the correlation with the changes of their properties has been performed for pure and doped with manganese iron oxide.