The catalytic properties of alkali metals (K and Na) co-adsorbed with carbon monoxide on Ni(111) surface are investigated by means of high resolution electron energy loss spectroscopy. This experiment highlights some important aspects of the catalytic reaction of CO2 formation, by clarifying the role of reactants, in particular of oxygen, and by exploring the kinetics of the reaction in different experimental conditions. The evolution of physisorbed carbon dioxide molecules with oxygen exposures is investigated as well, and, for the first time in this system, the formation of carbonates and chemisorbed carbon dioxide is shown. (c) 2005 Elsevier B.V. All rights reserved.