The effect of two amino-acid compounds, DL-alanine and DL-cysteine, on copper corrosion in an aerated 0.5 mol l(-1) HCl solution was studied by weight-loss measurements, potentiodynamic polarisation curves, and electrochemical impedance spectroscopy. A conventional benzotriazole (BTA) inhibitor was also tested for comparison. DL-cysteine was shown to be the most effective inhibitor among those tested inhibitors. Potentiodynamic polarisation results revealed that both the DL-alanine and DL-cysteine acted as an anodic inhibitor; however, DL-cysteine, in particular, was more effective, as it strongly suppressed anodic current densities. The improved inhibition efficiency of DL-cysteine in the 0.5 mol l(-1) HCl solution was due to its adsorption on the copper surface via the mercapto group in its molecular structure.