Two new mixed alkaline uranyl molybdates CsNa₃[(UO ₂)₄O₄Mo₂O₈] (1) and Cs₂Na₈[(UO₂)₈O₈(Mo ₅O₂₀)] (2) have been obtained by high-temperature solid state reactions. Their crystal structures have been solved by direct methods: Compound 1: triclinic, P1, a = 6.46(1), b = 6.90(1), c = 11.381.(2) Å, α = 84.3(1), β = 91.91(1), γ = 80.23(1)°, V = 488.6(2) ų, R₁ = 0.06 for 2865 unique reflections with |F₀| & 4σ_F; Compound 2: orthorhombic, Ibam, a = 6.8460(2), b = 23.3855(7), c = 12.3373(3) Å, V = 1975.2(1) ų, R₁ = 0.049 for 2120 unique reflections with. |F₀| ≥ 4σ_F. The structure of 1 contains complex sheets of UrO₅ pentagonal bipyramids and molybdenum polyhedra. The sheets have [(UO₂)₂O₂(MoO₅)] composition. Natrium and cesium atoms are located in the interlayer space. Cesium atoms are situated between the molybdenum, clusters, whereas natrium atoms are segregated between the uranyl complexes. The large Cs⁺ ions are localized between, the Mo₂O₉ groups and force the molybdenum polyhedra to rotate relative to the [(UO₂J ₂O₂(MoO₅)] sheets. Such rotation is impossible for U⁶⁺ polyhedra due to their rigid edgesharing complexes. The distance between the U⁶⁺ polyhedra vertices of neighboring layers is 3.8 Å, that allows the Na⁺ ion to be positioned between the uranyl groups. The crystal structure of 2 is based upon a framework consisting of [(UO₂)₂O₂(MoO₅)] sheets parallel to (010). The sheets are linked into a 3-D framework by sharing vertices with the Mo(2)O₄ tetrahedra, located between the sheets. Each MoO ₄ tetrahedron shares two of its corners with two MoO₆ octahedra in the sheet above, and the other two with MoO₆ octahedra of the sheet below. Thus four MoO₆ octahedra and one MoO₄ tetrahedron form chains of composition Mo₅O₁₈. The resulting framework has a system of channels occupied by the Cs⁺ and Na⁺ ions.