The metal cores of symmetrical sandwiches of alkaline-earth oxides have been found in the majority of cases to have a positive potential with respect to probes embedded in the oxide. These potential differences were much smaller in barium oxide than in either strontium oxide or in mixed (barium and strontium) oxides. They are attributed to local concentrations of positive ions (chiefly of strontium) which were produced near the metal cores owing to prolonged high temperature treatment during the pyrolysis of the carbonate.