We have measured the electronic absorption spectra of the U (III) and U (IV) ions in a LiCl–KCl eutectic melt at 450°C to understand their spectrochemical behavior in the context of pyrochemical process of spent nuclear fuel. The UV–VIS spectra of the U (III) ion consist of two main peaks in the range of 400–600nm which are attributable to the 5f 3 –5f 2 6d 1 transitions. The 5f–6d transitions of U (III) ion occurred at lower energy as low as 400–600nm range while that of Nd (III) and U (IV) ion occur at much higher energy over the normal UV range. For the U (III) ion, the intensities of 5f–5f transitions are about one order of magnitude larger than the 4f–4f transitions in lanthanides. And the 5f–6d transitions are about one order of magnitude smaller than in the 4f–5d transitions in the lanthanide ions. The electronic energy levels of the U (III), U (IV) and Nd (III) ions are estimated to understand their electronic transitions. The features of the electronic absorption spectra of U (III), Nd (III), U (IV) ions are reasonably explainable by their electronic structure. In-situ monitoring of the U (III)↔U (VI) redox reaction in a high temperature LiCl–KCl showed that the redox reaction is reversible.