Solid oxide electrolyzer (SOE) can operate in co-electrolysis mode of H2O and CO2 because of the high operating temperature and the nickel-based fuel electrode material. In this configuration, the fuel electrode is fed with a mix of steam and carbon dioxide that is converted through electrolysis reactions into a syngas that can be further upgraded to other carbon fuels. This technology can lead to interesting scenarios in the field of energy storage and CO2 reuse. SOE performance in terms of syngas quality and chemical equilibrium is presented in this study. An experimental test on a four-cell stack was performed to evaluate which are the effects of inlet gas composition and current density on the output syngas composition and on the thermal balance of the stack. Results were analyzed with the support of a thermodynamic model that aims to demonstrate that equilibrium condition is reached within the stack for all the involved reactions including reverse water shift. Also, we show that there is a direct relation between inlet and outlet compositions. For each composition, current density for thermoneutral condition was calculated obtaining values in the range of 377-493 mA/cm(2) depending on the composition. Such results permit to study SOE integration in a complete process for energy and CO2 storage. Copyright (C) 2015 John Wiley & Sons, Ltd.