The redox properties of tetrahalo uranyl(VI) complexes ([(UO2X4)-O-VI](2-) with X = Cl or Br) have been investigated in the ionic liquids [BuMelm][Tf2N] and [MeBu3N][Tf2N], where [BuMelm](+) and [MeBu3N](+) are respectively the 1-butyl-3-methylimidazolium and the tributylmethylammonium cations, both associated with the bis(trifluoromethylsulfonyl)imide anion [Tf2N](-). The results suggest that the reduction of [(UO2X4)-O-VI](2-) occurs via an Electron transfer - Chemical reaction - Electron transfer mechanism (ECE mechanism). We have shown by voltammetry that the first electron transfer is quasi-reversible and involves one electron, leading to the tetrahalo uranium(V) complex. This compound undergoes a chemical reaction and its stability depends both on the halide ligand and on the ionic liquid. The most stable complex is [(UO2Cl4)-O-V](3-) in [BuMelm][Tf2N]. We have assumed that the [BuMelm](+) cation interacts more strongly by H-bonding than the [MeBu3N](+) cation with the uranium complex, as it is the case for other anionic uranium species. Moreover, in presence of excess chloride ions, the consecutive chemical reaction is not observed anymore by cyclic voltammetry. (C) 2011 Elsevier B.V. All rights reserved.