The adsorption characteristics of methionine on mild steel surface in 0.5 M H2SO4 solution at a temperature of 25 +/- 0.1 degrees C have been investigated using electrochemical impedance spectroscopy, polarization resistance measurement and polarization curves measurement techniques. Constant phase element (CPE) has been used instead of double layer capacitance (C-dl) in circuit that models the double layer to represent the capacitive semicircle depression in the complex plane plots. The values of polarization resistance (R-p) from polarization resistance measurement technique agreed well with the sum of the distinct resistances (solution resistance, R, and charge transfer resistance, R-ct) from impedance technique. Adsorption of methionine on mild steel surface was found to obey the Langmuir adsorption isotherm with a standard free energy of adsorption (Delta G degrees(ads)) of -32kJ/mol. Localization of frontier molecular orbitals and the charge distributions were used to explain the electron transfer mechanism between the methionine molecules and the metal surface in contact. In addition, the local reactivity was examined by means of the Fukui indices. The distribution of the highest occupied molecular orbital (HOMO) in methionine molecule agrees with the atom that exhibits the greatest value of Fukui index (f(-), as a nucleophile), both indicate the region from where the methionine molecules can adsorb on the metal surface. Theoretical calculations were carried out at density functional theory (DFT) level using B3LYP functional with the 6-311**G(d,p) basis set for all atoms by Gaussian 03 W program. (C) 2007 Elsevier B.V. All rights reserved.