Mixed ionic- and electronic-conducting perovskite-type oxides are the state-of-the-art materials for high-temperature solid-state electrochemical devices such as solid oxide fuel cells (SOFCs), oxygen membranes, and sensors. Many of such materials are cobaltite-based oxides. Recently, double perovskites , where RE is a trivalent rare earth and the oxygen content varies in wide range, have received a great attention as attractive materials for such application. Many interesting phenomena, such as giant magnetoresistance, charge ordering, and metal-insulator transition, have been observed in these compounds. Powder samples of were synthesized by glycerol-nitrate method. Oxygen nonstoichiomentry of oxides was measured by the thermogravimetric (TG) method as a function of temperature in the range of in air. Total conductivity of aforementioned oxides was studied by the four-probe dc-method as a function of temperature in the range of in air. Polarization resistance of double perovskite cathodes was investigated by impedance spectroscopy in symmetrical cell of the type . “Metal-insulator” transition was found at in , whereas it was not observed in iron-doped sample due to the increase in oxygen content upon Fe-doping. At high temperatures, both double perovskites have almost the same total conductivity. Chemical interaction was found to decrease the performance of cathodes in YSZ-based SOFCs due to the chemical interaction between electrolyte and cathode materials, which significantly increases their polarization resistance. Behavior of total conductivity of oxides with temperature was explained by assuming small polaron charge transfer. The particularity of the latter is larger mobility of electron holes as compared with that of electrons. Increase in cathode performance was shown in the case of YSZ covered by the layer in comparison with pure electrolyte.