The synergistic effect at the interface of the bilayer Cu monomer or CuO dimer modified CeO2(1 1 1) catalyst for CO oxidation is investigated with density functional theory calculation. Important factors related to the synergistic effect such as interfacial ionicity, electron transfer, and charge partitioning of the Cu modifications are analyzed in detail for better understanding and control of this catalyst system. (C) 2011 Elsevier B.V. All rights reserved.