Using potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS), the inhibition ability of benzimidazole (BI), 2-methylthiobenzimidazole (2-MBI) and 2-chloromethylbenzimidazole (2-CBI) for mild steel in an HCl solution (1 mol L-1) was investigated. With an increase in the concentration, 2-MBI showed the highest inhibition efficiency (IE) at 1.24×10-3 mol L-1 , while the IE of BI and 2-CBI increased continuously within the investigated concentration range. The IE increased in the order of BI < 2-MBI < 2-CBI, which was further confirmed by the morphologies of the mild-steel surfaces after their immersion in 1M HCl with or without different inhibitors. Fitted adsorption isotherm curves suggested that the adsorption of the three inhibitors onto the mild steel in 1M HCl follow the Langmuir adsorption isotherm. Considering the effects of the temperature on the adsorption behaviour of the inhibitors, the apparent activation energy (Ea) of the mild-steel corrosion in the HCl solution with BI, 2-MBI and 2-CBI was calculated as (42.00, 48.28 and 67.93) kJ mol-1 , respectively.