Abstract: In solid oxygen ion conducting electrolytes for electrochemical cells based on magnesium oxide and calcium oxide, obtained by the addition of metal fluorides selected from elements in the groups of alkali metals and earth alkali metals to the host oxides of magnesium and calcium, conductivity values are obtained, which are comparable with those of stabilized zirconia, but the magnesium oxide and calcium oxide based oxygen ion conducting electrolytes have a superior thermodynamic stability and, therefore, can operate at much lower oxygen concentrations in comparison with other oxygen ion conducting electrolytes and without becoming noticeably electronically conductive.