The interaction of Ni(II) ions with nicotinamide and nicotinic acid in aqueous solution were examined by potentiometric, dc polarographic and spectroscopic techniques. The stoichiometry of the species reduced on a mercury electrode, and their electrochemical parameters were determined. In both systems the ML complex is reduced on the electrode at less negative values of potentials and more reversible than aqua-ion of Ni(II). The stability constants of the nicotinate and nicotinamide complexes were determined in 0.1 M NaClO(4) and 20 degrees C. It was shown that the polarographic technique is able to detect the complex species which are not seen in potentiometric titration and to determine its stability constants as well as potentiometric method. The relationship between the complex structure and its electrochemical reactivity is also discussed. (C) 2009 Elsevier B.V. All rights reserved.