The phase-shift method of the Frumkin adsorption isotherm of the over-potentially deposited hydrogen (OPD H) for the cathodic H-2 evolution reaction (HER) at the poly-Ni/0.05 M KOH aqueous electrolyte interface has been studied using cyclic voltammetric and ac impedance techniques. The behavior of the phase shift (0degrees less than or equal to -phi less than or equal to 90degrees) for the optimum intermediate frequency corresponds well to that of the fractional surface coverage (1 greater than or equal to theta greater than or equal to 0) at the interface. The phase-shift method, i.e., the phase-shift profile (-phi vs. E) for the optimum intermediate frequency, can be used as a new method to estimate and analyze the Frumkin adsorption isotherm (theta vs. E) of the OPD H for the cathodic HER at the interface. At the poly-Ni/0.05 M KOH aqueous electrolyte interface, the rate (r) of change of the standard free energy of the OPD H with theta, the interaction parameter (g) for the Frumkin adsorption isotherm, the equilibrium constant (K) for the OPD H with theta, and the standard free energy (DeltaG(theta)) of the OPD H with theta are 24.8 kJ/mol, 10, 5.9 x 10(-6) less than or equal to K less than or equal to 0.13, and 5.1 less than or equal to DeltaG(theta) less than or equal to 29.8 kJ/mol, respectively. The electrode kinetic and thermodynamic parameters (r, g, K, DeltaG(theta)) depend strongly on theta (0 less than or equal to theta less than or equal to 1).