To characterize electrosorption phenomena in well-defined interfacial conditions, impedance measurements with sulfate and perchlorate mixed electrolytes were performed on Pt(1 1 1). The diffusion impedance allowed us to detect the species coming from the solution and to evaluate the electrosorption valency. The main limitation in evaluating the electrosorption kinetics is not related to solution purity but to the electrolyte resistance, which is higher in sodium salts than in the corresponding acids. The 'usual' state seems related to a rather fast hydrogen electrosorption, but involves in some cases HSO4- dissociation. In sulfate solutions, the 'unusual' state is related to the fast electrosorption of a sulfate species, SO42- at high pH, HSO4- (or SO42- coming from HSO4-dissociation) at low pH. In perchlorate solutions (without sulfates) a simple electrosorption phenomenon, probably OH electrosorption, is coupled with another very fast step.