Impedance measurements for the interface between mild steel/hydrochloric acid as a function of cysteine (Cys) concentration were carried out for a wide enough frequency range. The validity of the impedance data was checked using the Kramers-Kronig (K-K) transforms. Despite the deviations at low frequencies for both transforms, imaginary-to-real and real-to-imaginary, a successful fit for the high frequencies was reached. The impedance parameters were determined from imaginary component of experimental impedance vs. logf diagrams using integration method and from complex plane plots fitting experimental results to an appropriate equivalent circuit. A compromise of parameters from two different plot methods was seen. (C) 2011 Elsevier Ltd. All rights reserved.