The electrochemical properties of neptunium, dissolved in the LiCl-KCl eutectic, were investigated by transient electrochemical techniques, such as cyclic voltammetry (CV) and chronopotentiometry on inert tungsten electrodes. It was shown that Np4+ is reduced to Np-0 by a two-step mechanism corresponding to the transitions of Np4+/Np3+ and Np3+/Np-0. In the 400-550 degrees C (673-823 K) temperature range, the diffusion coefficients of Np3+ and Np4+ were found to be of similar magnitude and equal to D-Np- = 9.47x 10(-4) exp(-23.6 x 10(3) /RT(K)) and D(Np)4+ = 2.14 x 10(-4) exp(-14.1 x 10(3) /RT(K)) cm(2) s(-1), respectively. The apparent standard potentials of Np4+/Np3+ and Np3+/Np-0 redox systems were determined to be E-*(Np4+/Np3+) 1.250 + 6.637 x 10(-4) center dot T (K) V versus Cl-2/Cl- and E-degrees*(Np3+/Np-0) = -3.250 + 7.251 x 10(-4) center dot T(K) V versus Cl-2/Cl-, respectively. The Gibbs free energies of formation in diluted solutions of NpCl3 and NpCl4 in the LiCl-KCl eutectic were determined to be Delta G(*degrees) = -936.54 + 0.20654 center dot T(K) and - 120.61 + 0.06405 center dot T (K) kJ mol(-1), respectively. Between 400 degrees C and 550 degrees C, the activity coefficients gamma(NpCl3) and gamma(NpCl4) range between 9 - 125 x 10(-6) and 2 - 10 x 10(-3), respectively. (C) 2007 Elsevier B.V. All rights reserved.