Six types of normal vibration for the bridge CO forming a (2x1)p2mg structure on Pd(110) were identified using high-resolution electron energy-loss spectroscopy (HREELS). Four types of vibrational mode were observed in the specular spectra, which are related to the tilted CO geometry along [001], i.e., a C O internal and a Pd CO external stretching mode, a hindered rotational and a hindered translational mode along [001]. The assignment was confirmed from the isotope shifts of a series of CO isotopomers. Two other vibrational modes, i.e., a hindered rotational and a hindered translational modes along [110], were observed in the off-specular spectra along [110].