Degradation processes of oxygen electrodes in solid oxide electrolysis cells (SOECs) were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Mossbauer spectroscopy. La0.58Sr0.4Fe0.5Co0.5O3-x (LSCF) anodes (oxygen electrode) were analyzed after different long-term operations durations of 1774, 6100 and 9000 h. The results were compared with a cell in the initial state. Besides the LSCF anode, the SOECs were composed of a Ce0.8Gd0.2O1.9 barrier layer between the anode and electrolyte, yttria-stabilized zirconia (YSZ) as electrolyte and Ni-YSZ as cathode (hydrogen electrode). Mossbauer spectra of the iron-containing anode were acquired in order to determine the alteration of the iron oxidation state and its local environment during operation. Mossbauer spectroscopy yields indirect information about the degradation mechanism, especially in combination with SEM, TEM, and XRD. XRD and TEM revealed the appearance of Co3O4 during the SOEC operation and SEM analyses confirmed the formation of SrZrO3 at the electrode/electrolyte interface. The spectral analysis confirmed the reduction of iron from Fe(IV) to Fe(III) in LSCF after long-term operation. The fraction of Fe(IV) in the electrode decreased with time and 18, 15, 13 and 11% were obtained for 0, 1774, 6100, and 9000 h of operation, respectively.