The relation between the phase-shift profile fur the intermediate frequencies and the Langmuir adsorption isotherm at the poly-Ir/0.1 M $H_2SO_4$ aqueous electrolyte interface has been studied using ac impedance measurements, i.e., the phase-shift methods. The simplified 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 pseudoca-pacitance $(C_phi)$. The comparison of the change rates of the $Delta(-phi)/{Delta}E;and;Delta{ heta}/{Delta}E$ are represented. The delayed phase shift $(phi)$ depends on both the cathode potential (E) and frequency (f), and is given by $phi=tan^{-1}[1/2{pi}f(R_s+R_F)C_P]$. The phase-shift profile $(-phi;vs.;E)$ for the intermediate frequency (ca. 1 Hz) can be used as an experimental method to determine the Langmuir adsorption isotherm $( heta;vs.;E)$. The equilibrium constant (K) for H adsorption and the standard free energy $({Delta}G_{ads})$ of H adsorption at the poly-Ir/0.1 M $H_2SO_4$ electrolyte interface are $2.0 imes10^{-4}$ and 21.1kJ/mol, respectively. The H adsorption is attributed to the over-potentially deposited hydrogen (OPD H).