Abstract: Complex impedance diagram on Bi₂O₃-Y₂O₃ system was investigated using AC impedance method. It was found that Ba₂O₃-based materials are characteristic of the Warburg impedance. The rate-limiting step of the electrode process at high Po₂ is the diffussion of oxygen atom into the solid electrolyte on the triple line. At low Po₂ the electrode process is controlled by diffusson as well as by concentration polarization, or controlled only by concentration polarization. Ba₂O₃-Y₂O₃ system characterized as a pure oxygen icn conductor at Po₂=1×1^-10～1×10⁵Pa.

Key words: Bi₂O₃-Y₂O₃3, AC impedance, oxygen ion conductor, Warburg impedance, constriction resistance