Two new mixed organic–inorganic uranyl molybdates, (C 6H 14N 2) 3[(UO 2) 5(MoO 4) 8](H 2O) 4 ( 1) and (C 2H 10N 2)[(UO 2)(MoO 4) 2] ( 2), have been obtained by hydrothermal methods. The structure of 1 [triclinic, P 1 ̄ , Z=1, a=11.8557(9), b=11.8702(9), c=12.6746(9) Å, α=96.734(2)°, β=91.107(2)°, γ=110.193(2)°, V=1659.1(2) Å] has been solved by direct methods and refined on the basis of F 2 for all unique reflections to R1=0.058, which was calculated for the 5642 unique observed reflections (| F o|⩾4 σ F ). The structure contains topologically novel sheets of uranyl square bipyramids, uranyl pentagonal bipyramids, and MoO 4 tetrahedra, with composition [(UO 2) 5(MoO 4) 8] 6−, that are parallel to (−101). H 2O groups and 1,4-diazabicyclo [2.2.2]-octane (DABCO) molecules are located in the interlayer, where they provide linkage of the sheets. The structure of 2 [triclinic, P 1 ̄ , Z=2, a=8.4004(4), b=11.2600(5), c=13.1239(6) Å, α=86.112(1)°, β=86.434(1)°, γ=76.544(1)°, V=1203.14(10) Å] has been solved by direct methods and refined on the basis of F 2 for all unique reflections to R1=0.043, which was calculated for 5491 unique observed reflections (| F o|⩾4 σ F ). The structure contains topologically novel sheets of uranyl pentagonal bipyramids and MoO 4 tetrahedra, with composition [(UO 2)(MoO 4) 2] 2−, that are parallel to (110). Ethylenediamine molecules are located in the interlayer, where they provide linkage of the sheets. All known topologies of uranyl molybdate sheets of corner-sharing U and Mo polyhedra can be described by their nodal representations (representations as graphs in which U and Mo polyhedra are given as black and white vertices, respectively). Each topology can be derived from a simple black-and-white graph of six-connected black vertices and three-connected white vertices by deleting some of its segments and white vertices.