Using cells with calcia stabilized zirconia as the solid electrolyte, the standard free energy of formation of stabilized Bi203 (fcc) was determined in the temperature range 750-950 K. The stability against reduction of stabilized Bi:O3 (fcc) is not increased with respect to pure 6-Bi203 (fcc). The corrosion of the calcia stabilized zirconia tube caused by Bi203 which took place along the grain boundaries, was investigated.