Density-functional theory has been used to study the CO oxidation reaction at model Au/MgO catalysts modified by either Na (an electron donor) or Cl (an electron acceptor) dopants. In agreement with experimental observations, Cl is found to act as a poison, making both the adsorption of O-2 and the formation of the CO . O-2 intermediate complexes that mediate the formation Of CO2 on these model catalysts more difficult. The poisoning effect of Cl has a long-ranged character, reaching at least two Au sites away from the adsorbed Cl atom. On the other hand. Na is found to be a promoter, enhancing both O-2 binding and CO . O-2) formation. Its promotive character is, however, local, involving the formation of strong Na-O bonds. (C) 2004 Elsevier Inc. All rights reserved.