The variation of the Cs 6s and the Cs 5p emission in He* and Ne* metastable deexcitation spectroscopy (MDS) as a function of the CO exposure indicates a demetallization of the Ru(0001)-(2x2)-Cs and the Ru(0001)-(root 3x root 3)R30 degrees-Cs surfaces upon CO coadsorption. This observation corroborates a (substrate-mediated) charge transfer from the Cs atom to the 2 pi* orbital of CO. With the Ru(0001)-(2x2)-Cs system even at CO saturation, MD spectra show emission associated with the Cs 6s state, indicating that the Cs atoms are not completely ionized, Exposing the (root 3x root 3)R30 degrees-Cs-pre-covered Ru(0001) to CO, surplus Cs of the first layer is displaced into a second layer In this way, CO molecules are able to be accommodated into the first layer, Desorbing this second layer Cs by heating the sample to 600 K produces a (2x2) structure with one Cs and CO in the unit cell as evidenced by MDS and low energy electron diffraction.