Abstract—Corrosion behavior of mild steel in 1 mol/L HCl with 2-chloromethylbenzimidazole (2-CBI) and sodium molybdate (Na2MoO4) as inhibitor was studied by using electrochemical impedance spectroscopy and potentiodynamic scanning. 2-CBI was proved an effective inhibitor for mild steel in 1 mol/L HCl, while Na2MoO4 did not. With a fixed 2-CBI concentration (100 mg/L) and increasing of Na2MoO4 concentration (50 to 300 mg/L), the inhibition efficiency (IE) of the 2-CBI/Na2MoO4 complex increased first and then decreased, and the synergistic parameters of the complex were always higher than 1. When the concentration of Na2MoO4 approaches to 150 mg/L, the IE of the complex showed a maximum value as 89.0%, which was higher than that of only 100 mg/L 2-CBI as inhibitor (81.8%). This result is considered as a compelling support for understanding the mechanism of the synergistic inhibition.