Summary The electrode behaviour of nickel(II) in aqueous solutions of iodides at mercury electrodes has been investigated using polarographic and voltammetric methods. It was found that at 25°C in moderately concentrated iodides NiI + is formed which is reduced with the standard rate constant of the order 10 −5 cm s −1 , at 50°C. At higher temperatures, 50–85°C, and very high ionic strength the formation of NiI 2 was found which is reduced under such conditions with the standard rate constant of the order 10 −3 cm s −1 . The detailed mechanism of both processes is discussed.