Results of combined experimental and theoretical investigations of elementary chemical reaction processes of CO-CO2 gas mixtures at nickel/yttria-stabilized zirconia (Ni/YSZ) solid oxide fuel cell (SOFC) model anode systems are presented. Temperature-programmed desorption and reaction measurements were performed in order to determine adsorption/desorption kinetic data as well as thermodynamic parameters for the CO/CO2/Ni and CO/CO2/yttria-stabilized zirconia (YSZ) heterogeneous reaction systems. From these data, an elementary kinetic reaction mechanism of the electrochemical CO oxidation at Ni/YSZ anodes was developed. Numerical simulations were performed for three different spillover mechanisms. Steady-state polarization curves and electrochemical impedance spectra were calculated, allowing for a direct comparison with experiments performed by Lauvstad et al. [J. Electrochem. Soc., 149, E506 (2002)]. Best agreement with the experimental data was obtained when assuming two consecutive charge-transfer steps from YSZ-O2- via YSZ-O- to Ni-O, the second step being accompanied by oxygen spillover over the three-phase boundary. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3505296] All rights reserved.