The effect of chemical environment on the neutralisation of ions with adsorbate covered surfaces was investigated on the example of H+ and rare gas ion scattering on chlorine covered Ag. Changes in coverage from the submonolayer chemisorption to chloride nucleation stage, strongly affect electron transfer. In the case of H+, Ne+ and Ar+ these changes induce an "unusual" non-linear, oscillating, coverage dependence of electron transfer probabilities, due to modifications in the surface electronic structure. In case of the more highly bound He, we assign the observed changes to modifications in quasi-resonant charge transfer rates involving the Cl 3s core level, because of the core level shifts induced by changes in chemical environment. (C) 2002 Published by Elsevier Science B.V.