Sulfur dioxide clusters (SO2) x of mean size 〈x〉 = 1.7 × 103 collide at a velocity of 1.6 km s-1 with a metal surface that has been pretreated with dilute salt solutions containing Li+, Na+, K+, Cl-, and Br- ions in different concentrations. In the scattered gas plume, positive and negative cluster fragments of the formal composition (SO2) x (Li+, Na+, K+, SO2 -) are detected. While the amount of charge observed in the various cationic channels correlates with the concentration of the respective cations in the solutions, no cluster fragments carrying chloride or bromide anions have been observed. This indicates, that the observed free charge carriers are not formed by a direct pickup of ions from the surface. Based on the assumption that the physical state for alkali adsorbates on metal surfaces is independent of the charge state of the adsorbing precursor, the findings are explained in terms of a known charge separation effect in cluster surface collisions involving neutrally deposited alkali adsorbates.