The degradation of anode supported cells was studied over 1500 h as function of cell polarization either in air or oxygen on the cathode. Based on impedance analysis, contributions of anode and cathode to the increase of total resistance were assigned. Accordingly, the degradation rates of the cathode were strongly dependent on the pO2; they were significantly smaller when testing in oxygen compared to air. Microstructural analysis of the cathode/electrolyte interface of a not-tested reference cell carried out after removal of the cathode showed sharp craters on the electrolyte surface where the LSM particles had been located. After testing in air, these craters flattened out and decreased in size, indicating a decreased three phase boundary length. In contrast, they remained almost unchanged after testing in oxygen giving an explanation for the observed smaller - mainly anode related - degradation rate.