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Second-harmonic generation and electrochemical techniques are applied to characterize the adsorption of Keggin type heteropolyanions (P- and Si-dodecatungstates) on polycrystalline platinum. Optical signal confirms polytungstates adsorption in the overall potential range (0.04−1.3 V vs RHE) even for low concentrations, when adsorption manifestations in cyclic voltammograms are negligible. Basically, the dependence of adsorption on electrode potential is typical for inorganic anions (increase of coverage with potential in the hydrogen and double-layer regions and weakening of adsorption in the oxygen region). An unusual interaction with hydrogen adatoms Had is discovered, and similarity with spillover-like phenomenon is mentioned. The adsorbate−platinum interactions depend on both potential and nature of adsorbed species. These observations are qualitatively interpreted in terms of partial charge transfer from Pt to tungstate which is equivalent to partial W(VI/V) reduction at potentials more positive than the equilibrium potentials of the corresponding W(VI/V) redox pairs in solution bulk (molecular UPD). A qualitative model is proposed for polytungstate interaction with adsorbed oxygen, which explains the observed hysteresis phenomena.