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 2x2 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 nu(C-O) stretch-mode frequency and the (root 3x root 5)R30 degrees 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 theta(CO)-->0 increases nearly linearly with theta(Cs) indicating CO desorption out of a two dimensional Cs gas in which the lateral interaction increases with theta(Cs).