The co-adsorption of CO and Na on Ni(111) was investigated by High-Resolution-Electron Energy-Loss-Spectroscopy. The measurements were performed both at room temperature and at 160 K and for several Na coverages, from a sub-monolayer to almost two monolayers. At room temperature the CO molecules dissociate above a critical Na coverage. Unexpectedly, the CO dissociation takes place also at 160 K. Low Na coverages induce a direct CO-CO reaction, whose products, CO2 molecules and C atoms, remain adsorbed on the Ni surface at low temperature. (C) 2004 Elsevier B.V. All rights reserved.