This work presents a study on the electrochemical behavior of neptunium, and in particular of the Np(III)/Np(0) redox system, in the molten LiCl-KCl eutectic. Neptunium solutions have been prepared by two different methods: from NpO2 dissolution into LiCl-KCl by carbochlorination (Cl-2(g) + C-(s)) at 550 degrees C and from metal Np oxidation from an Al - Cu - Np alloy by CuCl2. These neptunium solutions were investigated by cyclic voltammetry. A mixture of Np(III), Np(IV) species, and Np(V) soluble oxychloride was obtained from NpO2 dissolution by carbochlorination. In order to remove any oxygenated species from the melt, this solution was contacted with metal Zr, leading in our experimental conditions to a mixture of stable Np(IV) and Np(III) species. The second method used by Np oxidation from an Al - Cu - Np alloy gave a stable Np(III) solution. The study of the reduction reaction Np(III)/Np(0) has been performed by different electrochemical techniques. Apparent standard potentials of the Np(III)/Np(0) system at different temperatures were determined by chronopotentiometry at zero current technique [E'(o)(Np(III)/Np(0)) = -2.68 vs Cl-2(g)(1 atm)/Cl- at 450 degrees C]. Standard free energy, enthalpy, and entropy of formation, as well as the activity coefficient of Np(III) in LiCl-KCl, were also obtained. (c) 2006 The Electrochemical Society.