The complex oxide Sr1.95Fe1.4Ni0.1Mo0.5O6-x-5(Fx), where x = 0, 0.1, 0.2 and 0.3, as one of the most promising materials for electrodes of symmetrical electrochemical devices based on solid electrolytes is studied. The unit cell parameter increases monotonically with the addition of the fluorine-containing precursor in the synthesis of materials. The absence of fluorine traces in the sintered oxides was reliably confirmed by a combination of potentiometry using a fluorine-selective electrode and in situ mass spectrometry analysis of the gas media formed during heating the synthesized powders. It is suggested that the use of a fluorine-containing precursor makes it possible to increase the concentration of oxygen vacancies in the oxide, which positively affects its functional characteristics, such as the oxygen exchange rate with the gas phase and the oxygen electroreduction rate.& COPY; 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.