A double-solid electrochemical cell for accurately controlling oxygen concentration in oxides is presented. The inner cell, containing the sample, is totally surrounded by a second cell in which the oxygen partial pressure is kept as close as possible to the oxygen pressure inside the inner cell. This atmosphere protects the inner cell (including the cold end, electric, and gas inlets/outlets) from leaks. The control of oxygen partial pressure in both cells is done electrochemically using calcia-stabilized zirconia solid electrolytes. The system allows different temperatures for the sample and the solid electrolytes. Experiments show no detectable leaks. The double cell functions well down to about 100 K below the temperature range reported by others. This improvement is traced back to the leak-resisting nature of the present experimental system.