This dissertation examines the long-term stability of Ln2NiO4+δ-type rare earth nickelates with respect to their application as oxygen electrode materials in solid oxide fuel and electrolyzer cells. Special emphasis is put on electrode poisoning caused by volatile Cr- and Si-compounds, which is a well-known degradation mechanism that significantly shortens the operating-life of solid oxide cells. Several long-term experiments on dense samples and porous electrodes were conducted to investigate the detrimental effects of Cr- and Si-poisoning. The long-term stability was assessed by monitoring the oxygen exchange kinetics on dense nickelate samples using the dc-conductivity relaxation (CR) technique, as well as by measuring the area specific resistance of porous electrodes by means of electrochemical impedance spectroscopy (EIS). Dense samples of La2NiO4+δ (LNO) were tested at 700°C and 800°C for subsequent periods of 1000 h in dry and humidified oxygen/argon atmospheres, with and without the presence of a Cr- and a Si-source. Changes in surface morphology and chemistry were analyzed using atomic force microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and high-resolution scanning transmission electron microscopy with energy dispersive X-ray and electron energy loss spectroscopy. LNO demonstrates excellent stability under dry conditions even when a Cr-source is present. Increasing the humidity of the surrounding O2/Ar gas mixture, however, results in a rapid decline of the chemical surface exchange coefficient (kchem) of oxygen, which could be attributed to the decomposition of LNO into several secondary phases at the sample surface. Cr-poisoning was tested under realistic SOEC/SOFC operating conditions on symmetrical button cells with LNO, Pr2NiO4+δ (PNO) and LNO-Ce0.9Gd0.1O2-δ (LNO-GDC) composite electrodes at 800°C under open current conditions as well as under current load. Similar to the experiments on dense samples the cells were subjected to dry and humidified O2/Ar gas mixtures for periods of several hundred hours, with and without Cr being present. Additionally, cell tests with reference electrodes were conducted to investigate the influence of electrode polarization on Cr-poisoning. Extensive post-test analyses were performed using SEM- and STEM-EDX-EELS. The performance of all cells remained stable under dry conditions,even under current load. However, with humidity and in presence of Cr the cell performance degraded and the polarization resistance contribution of one electrode increased significantly. EIS-measurements and post-test analyses revealed a correlation between Cr-deposition and electrode performance.