A long-term test with a two-layer solid oxide cell stack was carried out for more than 20,000 hours. The stack was mainly characterized in a furnace environment in electrolysis mode, with 50% humidification of H-2 at 800 degrees C. The endothermic operation was carried out with a current density of -0.5 Acm(-2) and steam conversion rate of 50%. Electrolysis at lower temperatures (i.e., 700 degrees C and 750 degrees C) and fuel cell operation (with 0.5 Acm(-2) and fuel utilization of 50%) at 800 degrees C were also carried out (<2000 h each) for comparison. The voltage and area specific resistance degradation rates were similar to 0.6%/kh and 8.2%/kh after similar to 18,460 hours of operation. In total, the stack was operated above 700 degrees C for more than 20,000 hours. Impedance measurement and analysis showed that the increase of ohmic resistance was the main degradation phenomenon, while electrode polarizations were kept nearly constant before a severe burning took place in one layer. Ni-depletion in fuel electrodes was confirmed during post-mortem analysis, which was assumed to be the major degradation mechanism observed. The stack performance and degradation analysis under different working conditions, as well as the results of preliminary post-mortem analysis will be presented. (C) The Author(s) 2018. Published by ECS.