The inhibition effect of 3-amino-1,2,4-triazole-5-thiol (3ATA5T) was investigated in 0.5 M HCl on carbon steel (CS) by electrochemical impedance spectroscopy and potentiodynamic measurements at various concentrations and temperatures. Results showed that the correlation between experimental (inhibition efficiencies. Delta G(ads), E-a) and quantum calculation parameters (dipole moment, E-HOMO, E-LUMO). The high inhibition efficiency was declined in terms of strongly adsorption of protonated inhibitor molecules on the metal surface and forming a protective film. (C) 2011 Elsevier Ltd. All rights reserved.