Abstract The neutralisation by resonant charge transfer (RCT) of Li + ions back-scattered from a Cs/Al(100) surface is studied theoretically. The case of a single Cs adsorbate on a free-electron substrate is modelled and used to determine the size of the region around the Cs adsorbate where the RCT process is deeply modified. The perturbation introduced by the Cs adsorbate has a very long range character. For back-scattering in the keV energy range, the Li + neutralisation cross-section is typically around 700 a 2 0. Back-scattering from the region around the adsorbate is associated with large yields of excited Li 2p atoms. The size of the perturbation zone around each Cs adsorbate is strongly dependent on the process that is considered and on the projectile energy. The possibility of and difficulties in describing the case of a low coverage Cs/Al(100) surface using the results obtained in the case of a single adsorbate on the substrate are discussed.