International audienceDue to its acido-basic, complexing and redox properties, hydrogen peroxide has been widely used in nuclear research and industry. Particularly, this salt-free reagent has been highly considered for the stabilization of Pu(IV) in acid conditions and the precipitation of Pu peroxides as precursors for PuO2 preparation. The addition of H2O2 to Pu(VI) acid solutions allows its conversion into Pu(V) which disproportionates and allows the accumulation of Pu(IV).[1] Further addition of H2O2 may lead to Pu(III), or a mixture of Pu(IV) and Pu(III), through the generation of a Pu(IV) peroxo complexe which decompose in acid conditions.[1,2] The continuous addition of H2O2 to Pu(IV) acid solutions leads to the progressive formation of a brown peroxo complex which turns red before insoluble plutonium peroxide precipitate.[2,3] The structure of these complexes which have been postulated in 1949 still remains under debate. Since, only one Pu peroxo structure has been recently proposed and concerned a dimeric molecular compound prepared in alkaline conditions.[4] The current study focuses on the behavior of Pu(IV) aqueous solutions in slightly acid conditions in the presence of hydrogen peroxide. We particularly describe the first observation of a water soluble polynuclear peroxo complex of Pu(IV) which has, up to our knowledge, never been reported in the literature