The effect of cysteine (RSH), methionine (CH3SR), cystine (RSSR) and N-acetylcysteine (ACC) on the corrosion behavior of mild steel in 40% H3PO4 solution without and with Cl-, F-, Fe3+ and their ternary mixtures was studied using both potentiostatic and electrochemical impedance ( EIS) techniques under anodic and cathodic polarization conditions. The inorganic additives stimulate the overall corrosion reaction while the amino acids inhibit it with a predominant effect on the dissolution of iron. Both RSH and ACC are adsorbed according to Temkin's isotherm while adsorption of RSSR and CH3SR follows Frumkin and Langmuir isotherms respectively. The standard free energy of adsorption (Delta G (o)(ads)) was found to be in the order: RSSR > RSH congruent to ACC > CH3SR. The binary mixtures of Cl- or F- with RSH or CH3SR are the best inhibitors ( IE > 90%) while those containing ferric ions or blend I and amino acids are not good corrosion inhibitors. EIS measurements showed that the cathodic reaction, hydrogen evolution, is charge transfer controlled while the anodic one, iron dissolution, is a complex process.