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Interfacial behavior of cysteine(Cys)between mild steel and sulfuric acid solution as a corrosion inhibitor has been studied with electrochemical AC(alternating current)and DC(direct current)techniques at(25.0±0.1)℃.The AC impedance results were evaluated using equivalent circuits in which a constant phase element(CPE)has been replaced with double layer capacitance(Cdl)to represent the frequency distribution of experimental data.Changes in impedance parameters(charge transfer resistance and double layer capacitance)indicated that cysteine molecules acted by accumulating at the metal/solution interface.The fractional coverage of the metal surface(θ)Was determined using AC impedance results and it was found that the adsorption of cysteine on the mild steel sttrface followed a Langmuir isotherm model with a standard free energy of adsorption(△G0ads)of-35.1 kJ·mol-1. To clarify the type of interaction between mild steel surface and cysteine molecules with a molecular orbital approach,electronic properties,such as,the highest occupied molecular orbital(HOMO)energy,the lowest unoccupied molecular orbital(LUMO)energy,and the frontier molecular orbital tee~cients have been calculated. Energy gaps for the interaction of mild steel surface and cysteine molecules(ELUMOFe-EHOMOCys and ELUMOCys-EHOMOFe)were used to determine whether cysteine molecules acted as electron donors or electron acceptors when they interacv:A with the miId steel surface.The local reactivity was evaluated through the condensed Fukui indices.Theoretical calculations were carried out using the density functional theory(DFT)at B3LYP level with the 6-311++G(d,p)basis set for all atoms by Gaussian 03W program.