Electrochemical behaviour of tricyclopentadienyl uranium IV complexes (RCp)(3)UCl (RCp = RC5H4 with R = H; Me : CH3; tBu : (CH3)(3)C); TMS : (CH3)(3)Si) was investigated in THF with Bu4NPF6 electrolyte at different size platinum electrodes. Cyclic voltammetry was performed at a conventional size electrode, under unsteady-state diffusion conditions and linear scan voltammetry at a microelectrode, under steady-state diffusion conditions. The different species of uranium IV are reduced according to a reversible one-electron process. The half-wave potentials of the four U(IV)/U(III) systems have been determined and compared. The diffusion coefficients of the U(IV) complexes have been calculated also. For the oxidation reaction, the use of a microelectrode allows the kinetic complications, which are observed at the conventional size electrode, to be eliminated. Thus, the reversible one-electron oxidation process is characterized. It leads to the formation of a U(V) complex which is an unusual oxidation state for organometallic uranium complexes. Cyclic voltammetry shows the instability of (RCp)(3)UCl+ (U(V) complexes). For the (MeCp)(3)UCl, the oxidation mechanism agrees with the model described by Saveant for a reversible one-electron process coupled to a disproportionation reaction. This chemical reaction regenerates the U(IV) complex at the electrode. This result has been generalized to the other three uranium (IV) complexes.