In our previous investigation it has been established that cobalt, unlike iron or nickel, does not passivate in concentrated or even fumic nitric acid. The dissolution of cobalt in this aggressive medium is, however, significantly reduced by the formation of a hydrated nitrate salt layer adhering tightly to the metal surface. On the basis of the open-circuit corrosion potential measurements of cobalt electrode, in a wide range of nitric acid concentrations, and polarization curves, obtained in 11 mol dm-3 nitric acid using a regulating device with negative output resistance, along with electrode impedance results, it was proved that the hydration number of nitrate salt plays the crucial role in the electrochemical behaviour of cobalt in concentrated nitric acid solutions. The importance of the free water released through a decrease in the hydration number was clearly observed.