High-resolution electron energy loss spectroscopy has been used to study the nature of the Na/Ni(111) and K/Ni(111) bond at very low coverages. Measurements taken for very clean alkali layers provided, for the first time, the expected behavior of the alkali−substrate stretching energy as a function of the alkali coverage. Moreover, we found that the Na−Ni (K−Ni) bond was dramatically influenced by coadsorbed CO molecules causing a red shift of the Na−Ni (K−Ni) vibrational frequency. We argue that this effect could be at the base of discrepancies existing between the calculated and measured behavior of the vibrational energies as a function of the alkali coverage.