We have measured the electronic absorption spectra of the U(III) ion in LiCl–KCl eutectic melt at 450°C to understand its chemical 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. With the aid of UV–VIS spectroscopic tool, in-situ measurement of chemical reactions of the U(III) with oxide ion as well as neodymium oxide was successfully achieved. The U(III) ion forms insoluble uranium oxide phases by reacting with oxide ion and lanthanide oxides.