The reaction mechanism of heterogeneous CO oxidation on yttria-stabilized zirconia (YSZ), frequently used as electrolyte in solid oxide fuel cell (SOFC) composite anodes, was investigated employing density functional theory (DFT). The results demonstrate the possibility for an Eley-Rideal type CO oxidation reaction on the electrolyte surface without the need for a metallic catalyst if the vacant sites of YSZ are filled by externally supplied oxygen, either by dissociative adsorption of gaseous O-2 or via bulk oxygen atoms delivered by the SOFC cathode. Our results are consistent with the findings of recent experiments [J. Electrochem. Soc. 158 (2011) B5]. (C) 2012 Elsevier B.V. All rights reserved.