The specific anion adsorption accompanying the upd of Zn2+ ions on platinum electrodes has been studied by the radiotracer S-35 labelled sulphate species and Cl-36 labelled Cl(-)ions. It has been found that sulphate ions adsorb readily on the top of technique using Zn adatoms while the adsorption strength of Cl(-)ions on the Zn adlayer is significantly lower than that of HSO4- (SO42-) ions. Indirect radiotracer evidence obtained from competitive adsorption studies is presented concerning the relatively high adsorbability of H2PO4- ions. It follows from the experimental results obtained that the calculation of the mass and charge balance (involving the determination of the so-called electrosorption valence) requires the knowledge of the extent of the anion adsorption on the adatom layer.