High temperature electrolysis (HTE) of steam, CO2, and steam and CO2 for highly efficient generation of hydrogen, carbon monoxide as well as syngas was investigated for four solid oxide cell stacks, all supplied by different stack manufacturers. The SOCs employed within the stacks were planar, and electrolyte or electrode supported with an industrial size between 80 and 128 cm². A comprehensive electrochemical characterization of both stacks and individual cells within the stacks was conducted by means of electrochemical impedance spectroscopy and polarization curve measurement. Detailed performance analyses showed the highest efficiency when operating the stack under H2O electrolysis, followed by co-electrolysis and eventually CO2 electrolysis. Subsequently, the stacks were operated under reversible systemrelevant steady-state conditions, thus varying the working temperatures, the current density and the gas inlet flow. For that purpose both the conversion rate and fuel utilization were set to be between 70% and 80%. All stacks were operated for long-term periods of >1,000 h, during which degradation monitoring was applied. The results obtained within the present study allow a better understanding of the electrochemical processes that occur during reversible operation and especially HTE, and provide a guideline for optimized operation of a fully autonomous rSOC system.