Measurements of the complex impedance of the interface between mild steel and deaerated 1 M HCl solutions without and with the addition of different inhibitors, derivatives of benzimidazole, have been made in the frequency range of (0.1-9.5) x 10(4) Hz. Adequate structure models of the interfaces as well as some data on the kinetics of the partial processes are offered. It is found that the double-layer capacity as well as the adsorption capacity can not be presented by a frequency independent lumped element. The experimental data show a frequency distribution and therefore a modeling element with frequency dispersion behavior; a constant phase element has been used. At high inhibitor concentrations a second time constant is clearly observed in the impedance spectra. It is most probably connected with the adsorption process. Two time constants determined by the charge transfer and the adsorption of the inhibitor, respectively, can be readily outlined.