The effect of preadsorbed Sm atoms on CO adsorption has been studied over the Ru(001) surface, employing temperature-programmed desorption and photoemission spectroscopy. It was found that the presence of Sm atoms yields more tightly bound CO species compared with CO molecules adsorbed on clean Ru sites. The tightly bound CO species interact attractively with coadsorbed Sm atoms, and this attractive interaction leads to a formation of two-dimensional Sm + CO coadsorption islands on the surface. It seems that the Sm + CO coadsorption island gives additional adsorption sites, which interact very weakly with CO located upon the island. Upon heating the Sm + CO coadsorbed layer, the tightly bound CO located inside the Sm + CO island subsequently dissociated and oxidized Sm to form two-dimensional compounds.