Carbon dioxide electrolysis was studied in Ni/YSZ electrode supported Solid Oxide Electrolysis Cells (SOECs) consisting of a Ni-YSZ Support, a Ni-YSZ electrode layer, a YSZ electrolyte, and a LSM-YSZ O-2 electrode (YSZ=Yttria Stabilized Zirconia). The results of this study show that long term CO2 electrolysis is possible in SOECs with nickel electrodes. The passivation rate of the SOEC was between 0.22 and 0.44 mVh(-1) when operated in mixtures of CO2/CO = 70/30 or CO2/CO=98/02 (industrial grade) at 850 degrees C and current densities between -0.25 and -0.50 A cm(-1). The passivation rate was independent of the current density and irreversible when operated at conditions that would oxidise carbon. This clearly shows that the passivation was not caused by coke formation. On the other hand, the passivation was partly reversible when introducing hydrogen. The passivation may be a consequence of impurities in the gas stream, most likely sulphur, adsorbing on some specific nickel sites in the cathode of the SOEC. Activation can be carried out by hydrogen reacting with adsorbed sulphur to form the volatile compound H2S. Because adsorption of sulphur is site specific, only a part of the nickel sites were passivated and long-time operation of CO2 electrolysis in these Ni/YSZ electrode supported Solid Oxide Electrolysis Cells seems therefore feasible. (C) 2009 Elsevier B.V. All rights reserved.