Alternating current (AC) impedance measurements of mild steel/sulfuric acid interface in the absence and in the presence of various concentrations of cystine (Cys-Cys) have been carried out in the 100 kHz-10 mHz frequency range. The results revealed that Cys-Cys is a good and effective inhibitor for mild steel corrosion in 0.5 M H2SO4 and its percent inhibition efficiency changes with its concentration. Changes in impedance parameters indicated the adsorption of Cys-Cys on the mild steel surface, which was verified by scanning electron microscope (SEM) and atomic force microscope (AFM) photographs. Adsorption of Cys-Cys on mild steel surface was found to obey the Langmuir adsorption isotherm with a standard free energy of adsorption (Delta G(ads)degrees) of -33.2 kJ/mol. Energy gaps for the interactions between mild steel surface and Cys-Cys molecule were found to be close to each other showing that Cys-Cys owns capacity to behave as both electron donor and electron acceptor.