The inhibition effect of four amino acids towards the corrosion of copper in 0.5 M hydrochloric acid solution has been studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The used amino acids are aspartic acid (Asp), glutamic acid (Glu), asparagine (Asn) glutamine (Gin),. Both potentiodynamic and EIS measurements reveal that these compounds have some protective effect against copper corrosion in 0.5 M HCl solution. The inhibition efficiency of these compounds increases in the order Gin > Asn > Glu > Asp. The protection of the four amino acids increases with increasing their concentration. Addition of iodide ions into 10 mM Gln solution gives an improvement of the protection efficiency, reaching a maximum value of 93.74% for 5 mM KI. It is also suggested that the amino acids are physically adsorbed on the copper. The adsorption of used amino acids on copper surface follows Langmuir isotherm. (c) 2008 Elsevier B.V. All rights reserved.