The oxidation and passivation mechanism and the passive behavior of nickel in molten carbonate have been investigated with impedance measurements, quasi-stationary polarization curve measurements, and scanning electron microscopy. The oxidation of nickel probably proceeds according to a dissolution and reprecipitation process. The slowest steps in the reaction sequence are probably dissociation of carbonate and diffusion of the formed NiO to the surface. In the passive range, it is most likely that dissolution of Ni2+ proceeds after diffusion of Ni2+ through the oxide layer. The Ni2+ is formed at the metal/oxide interface. The slowest process is the diffusion of bivalent nickel ions through the passive scale. If only one reaction path is assumed, the formation of trivalent nickel ions probably takes place at the oxide/melt interface. This reaction is accompanied by the incorporation of an oxygen ion and nickel vacancy in the NiO lattice. The trivalent nickel ions and the nickel vacancy diffuse to the bulk of the oxide scale. The slowest step in this sequence is likely the dissociation of the carbonate ions and the subsequent incorporation of the oxygen ion in the NiO lattice.