The inhibition effect of 1,1'-thiocarbonyldiimidazole (TCDI) on the corrosion behaviors of mild steel (MS) in 0.5 mol . L-1 H2SO4 solution was studied with the help of potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and linear polarization resistance (LPR) techniques. The effect of immersion time on the inhibition effect of TCDI was also investigated over 72 It. For the long-term tests, hydrogen evolution with immersion time (V-H2-t) was measured in addition to the three techniques already mentioned. The thermodynamic parameters, such as adsorption equilibrium constant (K-ads) and adsorption free energy (Delta G(ads)) values, were calculated and discussed. To clarify inhibition mechanism, the synergistic effect of iodide ion was also investigated. The potential of zero charge (PZC) of the MS was studied by electrochemical impedance spectroscopy method, and a mechanism of adsorption process was proposed. It was demonstrated that inhibition efficiency increased with the increase in TCDI concentration and synergistically increased in the presence of KI. The inhibition efficiency was discussed in terms of adsorption of inhibitor molecules on the metal surface and protective film formation.