Arginine plays several important roles in cellular metabolism; it is able to generate NO, which is a vasodilator. Arginine is able to complex cations in aqueous solutions because it is an amino acid. This paper studies the behavior of arginine as a ligand in the complex formation with iron(II) and iron(III) in 1.00 mol kg(-1) NaClO4 as ionic medium and at 25 degrees C. In the case of iron(II) the experimental data obtained by means of electromotive force measurements of galvanic cell with a glass electrode can be explained by assuming the formation of only a complex (1:1), and its stability constant is determined. Experimental data for the system iron(III) arginine, obtained from electromotive force measurements carried out by employing glass and Pt redox electrodes, are explained by assuming the presence of mononuclear, polynuclear and mixed complexes with participation of OH-. The stability constants of the assumed species are obtained and a hypothetical mechanism for the formation of the predominant complex is proposed.