[en] Hydrates or hydroxides have been confirmed primarily for the hexavalent uranium. Of the tetravalent uranium, the compound UO₂·xH₂O is known, the water content of which varies according to the conditions of preparation and drying. Studies of its formation, preparation, structure, magnetic susceptibility and chemical reactions are reviewed. Recent investigations have shown that a hydrate of U₃O₈ probably does not exist and the compound designated in the literature as U₃O₈xH₂O is a hydrate of composition UO_2.86nH₂O (where n = 0.5 and 1.5). Investigations of this compound's formation, preparation, optical properties, thermal stability and solubility are reviewed. The existence of four hydrates or hydroxides of uranium trioxide has been confirmed. They are UO₃nH₂O, where n = 0.5, 0.8, 1 and 2. They may be prepared by breaking down UO₄·xH₂O by hydrolysis of uranyl salts or hydrothermally from UO₃. Hydrothermal investigations in the range 25⁰C to 330⁰C, resulting in a phase diagram for the UO₂²⁺-UO₃-H₂O system, are reviewed. (UK)