(La,Sr)MnO3-3YSZ interface chemistry in air-annealed and operated SOFC cathodes has been studied by high spatial resolution TEM/EELS. Major changes in Mn L-2,L-3 and O K ELNES were observed. A mixture of manganese 3+/4+ is found in the LSM bulk phase. In contact with LSM, zirconia forms a solid solution with dissolved lanthanum and manganese. Manganese is always divalent in this solid solution. After processing and annealing in air, the (La,Sr)MnO3-3YSZ interface shows no ELNES features that distinguish from the adjacent bulk phases. After single cell SOFC operation in air/hydrogen, a strong enrichment of Mn2+ at the cathode interfaces is found with Mn-O interfacial bond contributions. Due to the high Mn2+ level at the interface, the local oxygen vacancy concentration in zirconia at the interface is increased and the electron transfer for the oxygen incorporation in the electrolyte is made easier. Thus, in the operating composite cathode, oxygen incorporation from the gas into the electrolyte is promoted by a Mn2+ enrichment at the triple phase boundaries. Cathodic polarization in the operating single cell is the cause for the Mn2+ enrichment. Interface point defect modeling is used to evaluate interface defect concentrations at LSM/YSZ interfaces. Mn4+ interfacial clusters are the dominant interface defects at very high oxygen partial pressure, while the concentration of Mn2+ interfacial defects increases with decreasing oxygen chemical potential or under polarization. This finding is in agreement with the experimental ELNES results. Model and experiments suggest that cathodic oxygen exchange is promoted under oxygen deficient conditions. (c) 2006 Elsevier B.V. All rights reserved.