The adsorption of polyvinylimidazole (PVI) from aqueous solutions on gold and silica substrates has been studied using reflectometry and compared to that measured on colloidal silica by the solution depletion method. The adsorption trends were studied as a function of pH, monovalent electrolyte (KCl, KNO3), and divalent electrolyte (ZnSO4) concentrations. Uptake on gold compares very well with that on silica. The main objective of this paper is to show experimentally the importance of small electrolyte ions in the adsorption of PVI by measuring their electrical contribution in the charge balance of the adsorption layer. We shall use the adsorption data on colloidal silica, which compare also very well with those on the silica- and gold-modified wafers, and combine them with electrophoretic measurements to calculate and discuss small ion contribution. The results demonstrate that up to 80% of the charge in the interface is neutralized by small ions and, consequently, that they play a significant role in the free energy of polymer adsorption. PVI adsorption does not obey the principle of surface charge neutralization. The results show that, for the significantly charged polyelectrolyte (in acidic media at pH 7 and below), the main part of the adsorbed polymer charge is screened by its counterions. Conversely, at pH 8 and above, the weakly charged polymer cancels only a very small fraction of the high surface charge but the counterion of the surface (K+) is now largely in excess. With use of the gold and silica substrates, it is shown that when the ploymer is weakly charged (pH 8 and above), the increase of the negative surface charge leads to the displacement of the polyelectrolyte by small K+ ions. Our results are in agreement with current SFC modelization of the adsorption of polyelectrolytes but they illustrate more quantitatively the predominant contribution of small ions; they also explain several features of polyelectrolyte uptake with regard to the specific effect of the type and the concentration of the dissolved electrolyte, in particular, the occurrence of irregular isotherms and of endothermic adsorption reactions.