Performance of an La0.6Sr0.4Co0.8Fe0.2O3-CGO-Ag cathode (where CGO is Ce0.8Gd0.2O1.9) on a Ce0.8Sm0.2O1.9 electrolyte was studied by the three-probe AC impedance method at the steady state of various DC bias currents. The electrode consisted of a porous layer of La0.6Sr0.4C0.8Fe0.2O3-CG0 (30 wt.%) and a layer of Ag particles coated on the surface. The Ag coating was used to improve the oxygen exchange reaction activity. As a result, an interface conductivity greater than I S/cm(2) was obtained at 600 degreesC with a low activation energy. The dependence of the AC impedance on the frequency was aanalyzed in order to clarify the reaction steps in the oxygen reduction mechanism. (0 2002 Elsevier Science B.V. All rights reserved.