The inhibitive action of thiourea on the corrosive behaviour of ARMCO iron was investigated in deaerated 0.5 M H2SO4 solution, by means of electrochemical impedance spectroscopy. The inhibitor effectiveness increases with concentration, reaches a maximum (at about 1 mM) and then decreases. The adsorptive behaviour of thiourea on the electrode surface up to its peak follows a Frumkin-type isotherm with lateral repulsion, where the molecules are vertically adsorbed on the iron surface via the sulfur atom. Thiourea acts as a mixed inhibitor up to the critical concentration. It decreases the dissolution of iron and the hydrogen evolution reaction by blocking the electrode surface. The free energy of adsorption Delta G(ad) = -39 kJ mol(-1) and the attraction constant a = -4.4.