A new uranyl molybdate, Cs₂[(UO₂)O(MoO₄)] (1), has been prepared by high-temperature solid-state reactions. The structure has been solved by direct methods and refined to R₁ = 0.0284 for |F_O| ≥4σ_F The compound 1 is orthorhombic, Pca2₁, a = 12.018(2), b = 12.438(2), c = 17.917(3) Å, V = 2678.2(7) ų, Z = 12. The structure of 1 is based upon one-dimensional chains consisting of corner-sharing UO₆ and MoO ₄ polyhedra. The UO₆ square bipyramids share cis equatorial vertices to form zigzag chains that are further stabilized by bidendate MoO₄ tetrahedra. The chains have composition [(UO ₂)O(MoO₄)]^2- and are oriented parallel to the c axis. In the structure, adjacent chains form pseudo-2D-layers parallel to (010). The Cs⁺ cations are in between the pseudolayers. The composition-structure relationships in alkali metal uranyl molybdates can be rationalized from the viewpoint of dimensional reduction using the following assumptions: (1) representation of the A_xU_nMo _mO_y formula (n ≤ m) as a sum of UMoO₆ (= UO₂MoO₄), A₂O, and A₂MoO₄ (A = alkali metal): A_xU_nMo_mO_y = kA₂O + nUMoO₆ + (m-n)A₂MoO₄, where x = 2(k + m - n) and y = k + 2n + 4m; (2) analysis of the dimensionality of uranyl molybdate structural units and its changes upon incorporation of ionic reagents A₂O and A₂MoO₄ into a parent framework of UO₂MoO₄ that consists of 3-D array of U and Mo based polyhedra. The relationships between different compositions and structures may be visualized using the UO₂MoO₄-A₂O-A ₂MoO₄ compositional diagram. The observed trends of decreasing dimensionality of the uranyl molybdate units allow to specify on this diagram fields of dimensionality.