Single crystals of (NH₄)₄[(UO₂)₅ (MoO₄)₇] have been synthesized hydrothermally using (NH₄)₆Mo₇O₂₄, (UO₂)(CH₃^- COO)₂·2H₂O, and H₂O at 180 °C. The phase has been characterized by single-crystal X-ray diffraction using a merohedrally twinned single crystal: it is hexagonal, P6₁, a = 11.4067(5) Å, c = 70.659(5) Å, V = 7961.9(7) ų, and Z = 6. The structure is based upon an open framework with composition [(UO₂)₅(MoO₄)₇]^4- that is composed of UO₇ pentagonal bipyramids that share vertexes with MoO₄ tetrahedra. The framework has large channels (effective pore size: 4.8 × 4.8 Ų) parallel to the c axis and a system of smaller channels (effective pore size: 2.5 × 3.6 Ų) parallel to [100], [110], [010], [110], [110], and [110]. The channels are occupied by NH₄⁺ cations and H₂O molecules. The topological structure of the uranyl molybdate framework can be described either in terms of fundamental chains of UO₇ pentagonal bipyramids and MoO₄ tetrahedra or in terms of tubular building units parallel to the c axis.