The activation mechanism of the (ORR) on lanthanum strontium manganite (LSM) thin-film electrodes is investigated by examining the electrochemical behavior with impedance spectroscopy. To clarify whether surface segregation processes induced by electrochemical polarization are responsible for the change in catalytic activity of the perovskite electrode, in situ investigations with X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SIMS) were performed. The surfaces of the as-prepared thin-film electrodes, which were annealed at elevated temperatures during the preparation process, show an increased strontium surface concentration. The segregated SrO surface layer has a detrimental effect on the ORR reaction. Cathodic polarization decreases the strontium surface concentration while anodic polarization causes strontium accumulation at the electrode surface, which is proven by both SIMS and XPS in situ. A mechanism based on the incorporation of poorly conductive SrO from the electrode surface into the LSM lattice under cathodic polarization is suggested to be responsible for the observed activation process. (C) 2012 Elsevier Inc. All rights reserved.