A number of inhibitors were investigated, comparing in particular inhibitor efficiencies at room and higher temperature. The beneficial action of colloidal inhibitors depends on the adsorption of the inhibitor by the cathodic areas of the local cells or couples which are present on the surface of the metal. The assumption of the presence of such couples is supported by the presence of cathodic overvoltages and absence of anodic overvoltages. The higher iron oxides are reduced in spite of the presence of the inhibitor. The specific beneficial action of the inhibitor is the retardation of the evolution of hydrogen. There is a distinct parallelism between inhibitor action and hydrogen overvoltages.