The relation between the phase-shift profile for the intermediate frequencies and the Langmuir adsorption isotherm at the poly-$Pt/0.1;M;H_2SO_4$ aqueous electrolyte interface has been studied using ac impedance measurements, i.e., the phase-shift methods. The suggested interfacial equivalent circuit consists of the serial connection of the electrolyte resistance ($R_S$), the faradaic resistance $(R_F)$ and the equivalent circuit element $(C_P)$ of the adsorption pseudocapacitance $(C_varphi)$. The delayed phase shift $(varphi)$ depends on both the cathode potential (E) and frequency (f), and is given by $varphi=-tan^{-1}[1/2{pi}f(R_s+R_F)C_p]$. The phase-shift profile $(varphi;vs.;E)$ for the intermediate frequency (ca. 6Hz) can be used as an experimental method to determine the Langmuir adsorption isotherm (9 vs. E). The equilibrium constant (K) for H adsorption and the standard free energy $({Delta}G_{ads})$ of H adsorption at the poly-$Pt/0.1;M;H_2SO_4$ electrolyte interface are $1.8 imes10^{-4};and;21.4kJ/mol$, respectively. The H adsorption is attributed to the over-potentially deposited hydrogen (OPD H).