[en] By slow evaporation of solutions containing UO₂(ClO₄)₂ and an excess of HClO₄, single crystals of [UO₂(ClO₄)₂(H₂O)₃] (1) and [UO₂(H₂O)₅](ClO₄)₂ (2) were obtained and their structures were determined. From similar solutions prepared from stoichiometric amounts of UO₃ and perchloric acid, crystals of [UO₂(H₂O)₅](ClO₄)₂.2H₂O (3) were obtained. The trihydrate (monoclinic, P2₁/c, a = 545.44(1) pm, b = 1811.09(5) pm, c = 1032.46(2) pm, β = 90.016(1) ) consists of uranyl ions, which are coordinated by two monodentate perchlorate ions and three water molecules. The pentahydrate (monoclinic, P2₁/n, a = 529.35(2) pm, b = 1645.43(6) pm, c = 1480.18(6) pm, β = 99.847(1) ) contains uranyl ions coordinated by five water molecules. The same structural unit can be found in the heptahydrate, whose structure was re-determined (orthorhombic, Pbcn, a = 920.9(3) pm, b = 1067.9(3) pm, c = 1445.7(3) pm). In this structure, two molecules of water of crystallization are present. (Abstract Copyright [2003], Wiley Periodicals, Inc.)