The co-adsorption of CO and C, O on Na pre-covered Pd(1 1 1) surface are studied by density functional theory (DFT). CO adsorption is affected by the short-range Na-CO electrostatic interactions, especially the Na-O attraction, and the Na-induced long-range surface polarizability as well as the indirect interactions through substrates. Na stabilizes C and O adsorptions, particularly O adsorption, though both C-Pd and O-Pd bonds are elongated. Na reduces CO dissociation barrier by similar to 25%. The promotional effects are primarily ascribed to the short-range Na-O electrostatic interaction which stabilizes the transitional state. The Na-induced long-range effects are not significant and hardly promote CO dissociation. (C) 2011 Published by Elsevier B.V.