The author has examined the rate of reaction of the system thiocyan ate/sulphate and cyanide, by means of the current/anode potential curves obtained during the electrolytic oxidation of thiocyanate in an acid medium. Corresponding with the minimum current detectable under our experimental conditions, we find a potential which agrees well with that determined by a classical potentiométrie method, and checked chemically. Consequently, it can be used to predict the chemical reactions taking place. From these curves, the relation between the anodic potential, the intensity of the electrolytic current, and the activity of the SCN - ions has been deduced. These methods have been applied in a new method of investigating reactions in solution, viz. the potentiometry at constant current other than zero.