Reduction of H2O and CO2 as well as oxidation of H-2 and CO was studied in a Ni/YSZ electrode supported Solid Oxide Cell (SOC) produced at DTU Energy conversion (former Riso DTU). Even though these Ni/YSZ based SOCs were developed and optimized for fuel cell use, they can work as reversible SOCs in mixtures of H2O, H-2, CO2 and CO. From polarization (i-V) and electrochemical impedance spectroscopic characterization, it is evident that, electrochemical reduction of both CO2 and H2O occurs during coelectrolysis of H2O and CO2 in these Ni/YSZ based SOC. During co-electrolysis, the equilibrium of the water gas shift reaction is reached, and CO is therefore produced via the water gas shift reaction also. Significant differences during oxidation/reduction in H2O - H-2 and CO2 - CO mixtures were observed implying that different reaction mechanisms apply for the mixtures. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.076208jes]