The adsorption of CO on the Sm-precovered Ru(001) surface has been studied by means of high-resolution electron energy loss spectroscopy (HREELS). A highly inclined CO species with an extremely low C-O stretching [v(C-O)] frequency of similar to1060 cm(-1) was identified in the Sm and CO coadsorbed layer. This unusual bonding geometry with a severely weakened C-O bond indicates a strong chemical interaction with neighboring Sm atoms. The Sm-induced dissociation of CO was found to result from this highly inclined precursor state when the surface was covered with more than similar to0.4 monolayer (ML) of Sm or annealed above similar to300 K. Several intermediate states with v(C-O) modes in the range of similar to1120-1560 cm(-1), whose C-O bonds Were weakened but probably not tilted, also appeared along with a Sm-free CO species, depending on the coverages of both Sm and CO, as well as the thermal annealing temperature. Upon thermal annealing, a part of these intermediate states were converted into the highly inclined precursor state to dissociate, while the others desorbed without dissociation. In addition, a second-layer CO species with a high v(C-O) frequency of similar to216.0 cm(-1) was observed and adsorbed very weakly onto a cationic Sm atom in the coadsorbed layer.