A systematic study of the corrosion and passivation behaviour of cobalt in aqueous solutions of different pH was carried out. Open circuit potential measurements, polarization experiments and electrochemical spectroscopic (EIS) investigations were employed. The experimental results show that the metal surface is always covered by a native passive film which consists of CoO. The formation of the oxide film obeys a two-electron charge transfer process. The dissolution of the barrier film is controlled by the pH of the solution. In neutral and basic solutions the barrier film is stable. In these media a barrier film thickening with the formation of secondary layer is considered. In acidic solutions, the passive film is unstable and dissolves via a pure chemical process. The mechanism of the corrosion and passivation processes taking place at the electrode/electrolyte interface in the different solutions is discussed. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to confirm the electrochemical measurements and the suggested mechanisms.