Crystals of [NaU(Mo6P4O31H7)(2)] 5Na (H2O)(n) (NaUMo6) have been synthesized by slow evaporation of an aqueous mixture containing uranyl nitrate, sodium molybdate, phosphoric acid, and sodium dithionate. Single crystal diffraction of NaUMo6 reveals the assembly of {Mo6P4} clusters linked into one-dimensional chains with alternating Na+ and U4+ cations. To our knowledge, NaUMo6 is a unique example of Mo5+ based polyoxometalate associated with actinides. With the use of similar synthesis conditions but without uranium in the aqueous solution, [Na(Mo6P4O31H10)(2)] 5Na (H2PO4) (H2O)n (NaMo6) is obtained. NaMo6 is a sandwich type cluster which is built on the assemblage of two {Mo6P4} units linked by one sodium cation. Using small-angle X-ray scattering techniques and aqueous electrolyte based dissolution strategies, we can accurately observe chains of [UNa(Mo6P4O31H7)(2)]n(5)n, where n = 7 is the dominant soluble specie. Likewise, the dimeric form of [Na(Mo6P4O31H10)(2)](5) dominates the aqueous solution, revealing the structural units observed in the crystal structure are also stable in solution, under appropriate dissolution conditions.