We investigate the synergistic effect of iodide ions on the corrosion inhibition of manganese-14 steel in 0.5 M H2SO4 in the presence of cyclohexylamine using potentiodynamic polarization and linear polarization techniques. In 0.5 M H2SO4, the corrosion rates of the steel decrease when amine the concentration decreases, while the inhibition efficiencies increase. The addition of iodide ions enhances the inhibition efficiency considerably. The presence of iodide ions increases the degree of surface coverage, while at low concentrations cyclohexylamine forms an insoluble complex by undergoing adsorption. The synergism parameters (S-theta) calculated from surface coverage in the case of cyclohexylamine are found to be larger than unity. In addition, the adsorption of this amine on manganese-14 steel is found to follow a Langmuir isotherm. Thus inhibition occurs by adsorption.