We report a high-resolution electron-energy-loss spectroscopy study of the coverage dependence of the Cs-Ru stretch vibration at 300K. Surprisingly, the frequency of Cs-Ru stretch vibration shifts upward by about 30% with Cs coverage for 0≤θCs≤0.19. Only a quarter of this shift can be explained through lateral dipoledipole interaction. We discuss two further mechanisms that may cause the other part of the shift. A slight decrease of the stretch frequency for θCs>0.19 is associated with the metallization of the alkali-metal layer including a structural change.