AbstractIn this study, two short SOE stacks (6‐cells) were characterized in steam‐electrolysis mode. The first was based on Ni‐YSZ supported cells with LSCF‐based air electrodes. The second included LSC‐based air electrodes specifically developed within the SOFC600 project. The stacks were fed with a 90% steam, 10% hydrogen mixture, and characterized between 600 and 700 °C. For the LSCF‐based stack, an average cell voltage of 1.6 V was reached at –1 A cm–2 and 700 °C, corresponding to 60% steam conversion. The LSC‐based stack showed better performances, with 1.25 V at –0.8 A cm–2 and 700 °C. Durability tests were performed around the thermoneutral voltage (1.35 V) for resp. 2,400 (LSCF) and 1,500 h (LSC). The average stack degradation rates were resp. +2.8% (kh)–1 (LSCF) and +1.9% (kh)–1 (LSC). Twelve load‐cycles were also performed on the LSC‐stack with no apparent degradation. In addition, co‐electrolysis was performed between 750 and 850 °C by feeding the stack with a 60% H2O, 30% CO2, and 10% H2 mixture. Ninety‐five percent conversion was reached and the measured outlet syngas composition was close to that predicted by thermodynamics. Steam electrolysis tests were carried out in the same conditions for comparison. The stack performance in the co‐electrolysis mode was slightly lower than in the electrolysis mode.