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In a 0.02 M borax solution (pH 8.5), basic amino acids (arginine, lysine, and ornithine) react with Ni 2þ to form a monoligand complex that is reduced at a mercury electrode at about À 0.85 V vs. Ag j AgCl j KCl (3 M). At a long time scale (staircase voltammetry; scan rate < 50 mV s À1 ), the complex reduction is a catalytic (EC') process, the ratedetermining step being the regeneration of the reducible species by the reaction of the amino acid with free Ni 2þ . At a short time scale (differential pulse voltammetry or higher scan rate staircase voltammetry), the reaction rate is controlled by the diffusion of the complex. Although the same kind of complexation occurs with either basic amino acids or glycine, the last one does not induce a similar process. The peculiar effect of basic amino acids is due to the side chain that causes the ligand molecule to adopt a favorable orientation at the electrode surface. The differential pulse voltammetry peak current is proportional to the total amino acid concentration over the concentration range from 2 to 100 mM. Hence a voltammetric method for arginine determination in nutritional supplements was developed and validated using HPLC as reference method.