The effect of polarization treatment on the interface between (La,Sr)MnO3 (LSM) cathode and Y2O3-ZrO2 (YSZ) electrolyte in solid oxide fuel cells has been studied by electrochemical impedance spectroscopy and atomic force microscopy. It has been found that the polarization has a significant effect not only on the electrocatalytic activities of the LSM electrode but also on the topography and morphology of the interface between the LSM electrode and YSZ electrolyte. The formation of a convex ring on the YSZ electrolyte surface and its subsequent broadening under various polarization treatments indicate that O-2 reduction and oxidation reactions most likely occur at the O-2/LSM/YSZ three-phase boundary areas. The width of the three-phase boundary was estimated to be 0.15 +/- 0.05 mum. (C) 2004 The Electrochemical Society.