The coadsorption of Na∕ C O on Ni (111) has been investigated using high-resolution electron energy loss spectroscopy. Loss measurements showed that coadsorbed CO molecules dramatically affect the vibrational properties of Na adatoms. The Na-Ni vibration energy at 22 meV measured on the Na∕ Ni system shifted down to 12 meV for the C O∕ Na∕ Ni surface. This result was ascribed to a charge transfer from Na to CO. On the contrary, no appreciable shift of the Na-Ni stretching frequency was observed for the coadsorption of Na with oxygen.