An unusaul mixed-valence uranium(V,VI) oxyhydroxide, Na-5[U5O16(OH)(2)], was synthesized via reduction of UO22+ with zinc under hydrothermal conditions at 570 degrees C and 150 MPa. Its structure consists of extended sheets of edge-sharing U-VI pentagonal bipyramids and U-V square bipyramids. The overall anion topology is the same as that of the uranyl mineral sayrite, but the squares are populated by U-V(OH)(2)(3+) ions. The valence state of uranium was confirmed by X-ray photoelectron spectroscopy. A hydrothermal reaction without zinc under the same reaction conditions yielded a uranyl oxyhydroxide, Na-5[U5O17(OH)], with a new sheet structure. The sheet anion topology, which is closely related to that of the mixed-valence compound, contains chains of edge-sharing pentagons as well as dimers of corner-sharing squares.