The electrochemical behaviour of the Np3+/Np couple in the LiCl-KCl eutectic salt was investigated by electromotive force measurements, cyclic voltammetry and chronopotentiometry in the temperature region between 723 and 823 K. The standard redox potential of the Np3+/Np couple vs Ag/AgCl (1.00 wt %) was measured and given by the equation, ENp3+/Np degrees = -2.0298 + 0.000706 T, where E is in V and T in K. The electrode reaction of the Np3+/Np couple was almost reversible under the conditions studied. The diffusion coefficient of Np3+, DNp3+, in the LiCl-KCl eutectic melts between 723 and 823 K was represented by the equation, D-Np = 2.22 x 10(-6) - 6.88 x 10(-9) T + 5.60 x 10(-12) T-2 cm(2) s(-1). The adsorption and desorption peaks of Np at the Mo working electrode caused by underpotential deposition were also observed in the cyclic voltammograms, and the work function of Np was evaluated as 3.04 eV by peak analysis of the cyclic voltammograms.