L-Leucine is evaluated as a potential inhibitor for mild steel in acidic medium by galvanostatic polarization and potentiostatic polarization techniques. The electrochemical results were supplemented by scanning electron microscopy (SEM) and infrared studies (IR). The electrochemical polarization results show that L-leucine is most effective at 10 (1) M concentration at room temperature (298 K). The efficiencies were found to decrease with decrease in concentration and increase in temperature. Electrochemical results also show that L-leucine acts as a mixed type of inhibitor (blocks the cathodic and anodic sites to same extent) which is evident from insignificant shift of open circuit potential. Potentiostatic polarization data shows that they are passivating type of inhibitors. The effect of this inhibitor on anodic reaction is mainly attributed to physical adsorption of the additive on the anodic metal surface and the electron pairs on oxygen atoms. This additive exists in the protonated form (a positive charge on nitrogen atom) in the present acid medium. Therefore, on the cathodic sites, the interaction between additive and metal surface is thought to be electrostatic in nature. The results of SEM and IR data supplement the results obtained by electrochemical techniques. (C) 2008 Elsevier B.V. All rights reserved.