The Langmuir adsorption isotherms of the under-potentially deposited hydrogen (UPD H) and the over-potentially deposited hydrogen (OPD H) at the Pt(100)/0.5 M H2SO4 and 0.5 M LiOH aqueous electrolyte interfaces have been studied using cyclic voltammetric and ac impedance techniques. The phase-shift profile (-phi vs. E) for the optimum intermediate frequency can be used as an experimental method to estimate the Langmuir adsorption isotherm (theta vs. E) at the interfaces. At the Pt(100)/0.5 M H2SO4 electrolyte interface, the equilibrium constant (K) and the standard free energy (DeltaG(ads)) ofthe OPD H are 1.5 x 10(-4) and 21.8 kJ/mol, respectively. At the Pt(100)/0.5 M LiOH electrolyte interface, K transits from 1.9 (UPD H) to 6.1 x 10(-6) (OPD H) depending on the cathode potential (E) and vice versa. Similarly, DeltaG(ads) transits from -1.6 kJ/mol (UPD H) to 29.7 U/mol (OPD H) depending on E and vice versa. The transition of K and DeltaG(ads) is attributed to the two distinct adsorption sites of the UPD H and the OPD H on the Pt(100) surface. The UPD H and the OPD H on the Pt(100) surface act as two distinguishable electroadsorbed H species. The UPD H and the OPD H at the Pt(100) interfaces are the independent processes depending on the H adsorption sites rather than the sequential processes for the cathodic H-2 evolution reactions. The criterion of the UPD H and the OPD H is the H adsorption sites and processes rather than the H-2 evolution reactions and potentials.