This study gives evidence that the time-dependent performance changes in anode supported cells for intermediate-temperature solid oxide fuel cells is essentially influenced by the mixed ionic-electronic conducting (MIEC) cathode. The impedance spectra recorded during 700 h of operation at 750 degrees C were interpreted using an appropriate equivalent circuit model by (i) a distribution of relaxation time analysis followed by (ii) a complex nonlinear least squares fit. Four electrode polarization processes were separated by selective experimental parameters. The cathodic part, initially the smallest, is only discovered among the anodic contributions by a change in fuel gas composition from H-2-H2O to CO-CO2 and increases by 310% (15 m cm(2) at 11 h, 62 m cm(2) at 700 h). A Sr (and Co) depletion of the MIEC cathode composition La0.58Sr0.4Co0.2Fe0.8O3-delta possibly caused this degradation. The anodic polarization has a proportion of 92% at the start and decreases to 73% (168 m cm(2) at 11 h, 173 m cm(2) at 700 h). The anode charge-transfer reaction initially causes 60% of the total polarization losses and 50% after 700 h. This is assigned to a change in the triple phase boundary and/or a degradation in ionic conductivity in the anode functional layer. The gas diffusion polarization remains constant at 58 m cm(2).