Nickel species are characterized under molten carbonate fuel cell acidic conditions, and under more basic levels in the presence or the absence of an oxygen partial pressure. Successive steps relative to the potential of a nickel foil under open-circuit conditions are analysed in these media. A voltammetric study is developed at a nickel or a gold electrode. Experimental evidence is given for an electrochemical intercalation-deintercalation process involving lithium ions in the NiO lattice. The difference between a fresh and an aged gold electrode is described. An electrocatalytic process relative to lithiated species is analysed under an argon atmosphere with the presence of oxide species.