Electrochemical decomposition of CO2 and CO gases using a porous cell of Ru-8 mol% yttria-stabilized zirconia (YSZ) anode/porous YSZ electrolyte/Ni-YSZ cathode system at 400-800 degrees C was studied by analyzing the flow rate and composition of outlet gas, current density, and phases and elementary distribution of the electrodes and electrolyte. A part of CO2 gas supplied at 50 ml/min was decomposed to solid carbon and O-2 gas through the cell at the electric field strengths of 0.9-1.0 V/cm. The outlet gas at a flow rate of 3 ml/min included 61-63% CO2 and 37-39% O-2 at 700-800 degrees C and the outlet gas at a flow rate of 50 ml/min included 73-96% (average 85%) CO2 and 4-27% (average 15%) O-2 at 800 degrees C. On the other hand, the supplied CO gas was also decomposed to solid carbon, O-2 and CO2 gases at 800 degrees C. The fraction of outlet gas at a flow rate of 50 ml/min during the CO decomposition at 800 degrees C for 5 h was 11-36% CO, 59-81% O-2 and 2-9% CO2. The detailed decomposition mechanisms of CO2 and CO gases are discussed. Both Ni metal in the cathode and porous YSZ grains under the DC electric field have the ability to decompose CO gas into solid carbon and O2- ions or O-2 gas. (C) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.