The behaviour of nickel species was investigated in Li + Na + K carbonate eutectic using thermodynamic data obtained at 550-750 degrees C. Potential-oxoacidity diagrams were established to predict the stability domains of Ni, Ni(II) and Ni(III) species. NiO species can be dissolved in either oxoacidic media (Ni2+) or oxobasic media (NiO22-). Three nickel systems NiO/Ni, Ni2+/Ni and Ni(III)/Ni(II) were characterized by electrochemical techniques at 650 degrees C under P(CO2)=1 atm. The solubility and solubility product of NiO were determined under these conditions.