[en] The influence of H2S on the corrosion of iron and the corrosion prevention mechanism of an inhibitor was investigated with a differential capacitance measurement and a weight loss measurement method. The results show that H2S accelerates both the anodic iron dissolution and the cathodic hydrogen evolution in most cases. However, H2S acts as an inhibitor of the corrosion of iron under certain special conditions. An EIS method is proposed to explain the ability of inhibitors