The analysis of mixed ionic electronic conducting (MIEC) cathodes with respect to operation temperature and time is essential for a target-oriented development of anode-supported solid oxide fuel cells (ASCs). This study tracks both issues by impedance spectroscopy on a high-performance cathode with the composition La(0.58)Sr(0.4)Co(0.2)Fe(0.8)O(3-delta) (LSCF). A wide set of impedance spectra were sampled at 600,750 and 900 degrees C over the entire operation time of 1000 h. The identification and quantification of the individual anodic and cathodic contributions to the polarization losses of an ASC were enabled by an appropriate equivalent circuit model. For this purpose, the impedance data sets were evaluated subsequently by (i) a DRT (distribution of relaxation times) analysis followed by (ii) a CNLS fit. The cathodic polarization resistance is attributed to the oxygen surface exchange and the bulk diffusion of oxygen ions and is described by a Gerischer element. The anodic polarization resistance is described by a Warburg element and two RQ elements according to physical origins. The thorough analysis of all data sets leads to the surprising outcome that the cathode degradation is most pronounced and moreover, increases with decreasing temperature. After 1000 h of operation, the cathode polarization resistance raised steeply from 0.012%/h at 900 degrees C over 0.28%/h at 750 degrees C to 1.49%/h at 600 degrees C. These latest findings will have far-reaching implications for the development of MIEC cathodes. (C) 2010 Elsevier B.V. All rights reserved.