Crystals of a new framework uranyl molybdate, [(C₂H ₅)₂NH₂]₂[(UO₂) ₄(MoO₄)₅(H₂O)](H₂O), were synthesized from a solution of UO₂(CH₃COO)₂ · 2H₂O, MoO₃, diethylamine and HCl in H ₂O at 220°C. The structure was studied at -127°C. The compound is hexagonal, space group P6₅22, a = 11.3612(13), c = 52.698(8)Å, V= 5890.7(13)ų, Z =3. The structure has been refined to R₁ = 0.050 (wR₂ = 0.100; S = 0.985) on the basis of 3103 unique observed reflections. The structure consists of a three-dimensional framework of composition [(UO₂) ₄(MoO₄)₅(H₂O)]_2- that consists of UO₇ pentagonal bipyramids that share equatorial corners with MoO₄ tetrahedra. The framework contains a three-dimensional system of channels. The first system of channels is parallel to [001] and has the dimensions 7.0 × 7.0 Å, which result in a crystallographic free diameter (effective pore width) of 4.3 × 4.3 Å (based on an oxygen radius of 1.35 Å). Other channels run parallel to [100], [110] and [010] and have dimensions 5.2 × 7.1 Å (giving an effective pore width of 2.5 × 4.4Å). One symmetrically unique [(C₂H ₅)₂NH₂]⁺ cation and H₂O molecule reside in the framework cavities. The channels parallel to [001] are chiral and are centered about a 6₅ screw axis. The uranyl framework with U:Mo = 4:5 in the structure of [(C₂H₅) ₂NH₂]₂[(UO₂)₄(MoO ₄)₅(H₂O)](H₂O) represents the third known type of chiral uranyl molybdate framework. Its relationships to the frameworks with U:Mo = 5:7 and 6:7 can be understood in terms of fundamental chains that have related but yet different topological structure.