The chemisorption of CO on Sm-covered Ru(001) and SmRu surface alloy has been investigated by XPS and TPD techniques. Sm-induced CO dissociation occurs on the Ru(001) surface. Sm is partly oxidized and agglomerated due to the dissociation of CO. Two molecular CO states are found: one is a chemisorbed CO on Ru atoms and the other is a weakly adsorbed CO associated with Sm2Ox. The latter is characterized by a new intense feature at 538.5 eV in the O 1s spectrum. The top layer of the SmRu surface alloy on Ru(001) is decomposed due to the selective oxidation of Sm by oxygen from CO. The CO TPD from the SmRu surface alloy film shows an additional feature at 345 K that is attributed to CO molecules from Rusingle bondSm2Ox interfacial sites.