Hydrogen adsorption isotherms, evaluated by combination of cyclic voltammetry and chronoamperometry, are reported on Pt(111) and Pt(100) surfaces in 0.1M HClO 4 . We found that at E>0.05V Pt(111) and Pt(100) are only partially covered by the adsorbed hydrogen (H ad ). On both surfaces, a full monolayer of the adsorbed hydrogen is completed at −0.1V, i.e. the adsorption of atomic hydrogen is observed in the hydrogen evolution potential region. We also found, that the activity of the hydrogen oxidation reaction is mirrored by the shape of the hydrogen adsorption isotherms, implying that H ad is in fact a spectator in the HOR.