The interaction of nicotinamide with tryptophan in the presence and absence of nickel(II) in 0.1 M ammonium tartrate (pH 8.94) has been examined by voltammetric techniques. Tryptophan gave a catalytic hydrogen peak at - 1.12 V versus an Ag |AgCl|saturated KCl reference electrode while nicotinamide formed two peaks at - 1.42 and - 1.57 V, respectively. In the absence of nickel(II), when the tryptophan concentration was increased, the peak of nicotinamide at - 1.42 V increased gradually while its peak at - 1.57 V decreased. It can be concluded that tryptophan plays an important role in the reduction mechanism of nicotinamide at the mercury electrode surface. Voltammetric results show that the nickel(II)-tryptophan and nickel(II)-nicotinamide complexes on the mercury electrode surface reduce at - 0.44 and - 0.94 V, respectively. These complexes are reduced at a more positive potential than that of the Ni(II)tartrate ( - 1.18 V). From electronic spectral data of the complexes, their stoichiometries of 1:1 (metal/ligand) in aqueous medium are determined. The interactions between tryptophan and nicotinamide were also studied in the presence of nickel(II). Excess nickel(II) causes changes in the voltammetric reduction peaks of nicotinamide and tryptophan and the formation of binary complexes. The nickel(II)-tryptophan (log β = 2.3) complex is more predominant than that of nickel(II)-nicotinamide (log β = 2.2) complex. This probably results from the binding sites and the size of the catalyst. Further, when nicotinamide was added to a solution containing the nickel(II)-tryptophan complex, or tryptophan to the cell including the nickel(II)-nicotinamide complex, the formation of a mixed-ligand complex was not seen. © 2001 Elsevier Science B.V. All rights reserved.