The electrochemical behaviour of nickel and its oxides in molten carbonates is very complex because of the continuous growth of the NiO layer and the material losses during the oxidation and reduction processes. Thus, cyclic voltammograms have been realised in molten Li2CO3+Na2CO3 at 650°C at different electrodes (gold covered by NiO, fresh and aged nickel); they allowed us to identify, at different scan rates, the reactions associated with the peaks observed. In particular, thanks to a planar nickel disc electrode, with a well-defined surface, the principal reduction peak has been attributed to the transformation of NiO into Ni. Other reduction peaks, occurring at lower potentials, are probably due to the same reaction and their presence to phase transitions. Evidence was given for the existence of a Ni/Ni2+ system and the formation of Ni(III), most probably NaNiO2, by the oxidation of NiO at relatively high potentials.