The tetrachlorouranium(VI) complex is formed in [ Bmim][Tf2N] and [MeBu3N][Tf2N] from a uranium( VI) solution in the presence of a stoichiometric quantity of chloride ions. The [(UO2Cl4)-O-VI](2-) absorption and emission spectra show bands splitting in comparison with the [(UO2)-O-VI](2+) spectra, as observed in the solid state, organic solvents, and chloroaluminate-based ionic liquids. The fluorescence lifetime of [UO2Cl4](2-) in [MeBu3N][Tf2N] is 0.7 +/- 0.1 mu s. The reduction potential of this complex is -1.44 and -1.8 V vs Ag/Ag+ respectively in [ Bmim][Tf2N] and [MeBu3N][ Tf2N] and does not depend on the chloride concentration. The mechanism proposed for the redox process is a monoelectronic reduction to form [(UO2Cl4)-O-V](3-), followed by a chemical reaction. The tetrachlorouranium( V) complex seems more stable in [ Bmim][Tf2N] than in [ MeBu3N][ Tf2N]. The electrochemical analysis put in evidence specific interactions of the ionic liquid cation with the uranium anionic species.