Photoemission spectroscopy (Ru 3d/C 1s and O 1s regions) has been used to investigate the formation of carbonate species on the Sm- and SmO x-covered Ru(001) surfaces at small exposures of CO. The results indicate that the reaction of ∼1 L CO with disordered Sm layers on the Ru surface at 105 K produces surface carbonate and weakly bound CO2 via the Boudouard reaction (2COg → CO2,ad+C ad). The latter species desorbs below 250 K, whereas the former is stable up to 500 K. The surface carbonate species is detected by the O 1s features as well as the C 1s feature developed in the Ru 3d region upon CO exposure. The CO reaction with the SmOx layer also leads to the formation of surface carbonate. The generation of carbonate on Sm and SmO x films seems to be related to the ionization of Sm atoms and the defects present in the partly oxidized Sm, respectively.