High-resolution electron energy loss spectroscopy has been used to study CO adsorption on clean and alkali-modified Ag single layers deposited on Ni(111) and Cu(111) surfaces. An increasing of the CO dissociation rate was obtained for the alkali-doped Ag/Ni(111) interface. On the contrary, for a monolayer of Ag on Cu(111), alkali adsorption causes an increasing of the CO dissociation barrier. For higher Ag thickness (10 layers), the alkali-modified surface strongly reacts with water molecules.