The electrochemical behaviour of metallic cerium and cerium dioxide is analysed in molten Li2CO3-Na2CO3 under the anodic and cathodic conditions used for the molten carbonate fuel cells (MCFC). Chronopotentiometric measurements on metallic cerium rods or foils show the influence of CeO2 growth and the deviation from its stoichiometric composition. A study of the voltammetric characteristics of dissolved cerium at a gold electrode, of metallic cerium and cerium oxide is carried out in both oxidising and reducing MCFC atmospheres. Experimental evidence is given for the existence of a solid-solid cerium system: Ce2O3(s)\CeO2(s) and a system relative to dissolved cerium species Ce2O3(l)\CeO2(l). These systems are in agreement with thermodynamic predictions. Other more complex phenomena involving interactions with alkali species are discussed.