In this study, adsorption of 2-amino-5-mercapto-1,3,4-thiadiazole (2A5MT) on mild steel (MS) surface in 0.5M HCl solution and its corrosion inhibition effect was studied in both short and long immersion times (over 120 h). For this purpose, a series of techniques, such as potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), thermogravimetric analysis (TGA), surface photographs, hydrogen evolution (V-H2-t) and change of open circuit potential with immersion time (E-ocp-t) were utilized. The values of activation energy for MS corrosion and the thermodynamic parameters, such as adsorption equilibrium constant (K-ads), free energy of adsorption (Delta G(ads)), adsorption heat (Delta H-ads) and adsorption entropy (Delta S-ads) values were calculated and discussed. The potential of zero charge (E-pzc) of MS in inhibited solution was studied both after short and long immersion times by EIS method, and a mechanism of adsorption process was proposed. Results showed that 2A5MT performed excellent inhibiting effect for the corrosion of MS in 0.5 HCl solution and inhibition efficiency is higher than 99% after 120 h at 1.0 x 10(-2) M. The high inhibition efficiency was discussed in terms of strongly adsorption of inhibitor molecules on the metal surface and forming a protective film. Surface photographs showed a good surface coverage on the metal surface. TGA results indicated that the surface inhibitor film has relatively good thermal stability. (c) 2007 Elsevier B.V. All rights reserved.