The cathodic reaction mechanism of a solid oxide fuel cell (SOFC) was investigated for an electrode-electrolyte system of Lao(0.6)Ba(0.4)CoO(3) (LBC)-BaCeO3, under both the O2-conducting and H+/O2- mixed-ionic conducting conditions. AC impedance measurements were carried out, and the electrode interfacial conductivities were calculated. The experimental results revealed that the processes dominating the electrode resistance for the O2- conducting and the H+/O2- mixed-ionic conducting conditions are different. It was also found that the process dominating the electrode resistance changes at 700 degrees C under H+/O2- mixed-ionic conducting conditions. The process dominating the electrode resistance above 700 degrees C is Postulated to be the reaction O-LBC + H-LBC --> OHLBC. LBC showed high-performance cathode characteristics for a SOFC using BaCeO3 electrolyte.