The surface chemistry of operating model SOFC cathodes with LSM catalyst and YSZ electrolyte was studied in-situ by photoelectron microscopy. The electrochemical activation of SOFC cathodes under cathodic biasing or polarization was explained by the finding that partially reduced manganese oxide species rapidly spread from the triple phase boundary over the electrolyte surface, thereby providing high electronic conductivity in the zirconia surface scale and promoting the direct incorporation of oxygen from the gas into the electrolyte. (C) 2008 Elsevier B.V. All rights reserved.