The interaction of Cs and CO was studied for submonolayer coverages of Cs on Ru (0001) by means of high-resolution electron energy-loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and low-energy electron diffraction (LEED). From the HREEL spectra and the 2×2 LEED pattern it is concluded that two-dimensional islands of Cs+2CO stoichiometry are formed first. For high CO doses CO adsorbs on the bare Ru surface between the islands as concluded from the v(C-O) stretch-mode frequency and the (√3×√3)R30℃ LEED pattern. Besides some local interaction between CO and Cs, there is strong evidence found for a long range, through substrate interaction. Our data can be understood in terms of a charge redistribution model; Cs donates a given amount of charge to the surface which is then backdonated to and shared between the adsorbed CO molecules within the (Cs+2CO)-islands and outside of them on the bare surface. The desorption temperature of CO with θCO→0 increases nearly linearly with θCs indicating CO desorption out of a two dimensional Cs gas in which the lateral interaction increases with θCs .