We prepared neptunium chloride complexes electrochemically and performed spectroelectrochemical measurements of neptunium ions in LiCl-KCl eutectic melts at 450°C, where the neptunium concentrations studied were in the range of 0.1-1 mM. We observed a highly absorbing f-d transition band of Np 3? at 383 nm, which was used to determine an formal potential of the Np 4? |Np 3? redox couple (E°0 = 0.45 V vs. Ag|Ag ? ) with a solution of concentration as low as *0.1 mM. This result agreed well with the value (E°0 = 0.42 V vs. Ag|Ag ? ) determined by cyclic voltammetry with a *15 mM solution.