We have synthesized and structurally characterized three pridylethylidene-functionalized diphosphonate-containing polyoxomolybdates, [{(MoO3)-O-VI}(2){(Mo2O4)-O-V}{HO3PC-(O)(CH2-3-C5NH4)PO3}(2)](6-) (1), [{(Mo2O6)-O-VI}(2){(Mo2O4)-O-V}{O3PC(O)(CH2-3-C5NH4)-PO3}(2)](8-) (2), and [{(Mo2O4)-O-V(H2O)}(4){O3PC(O)(CH2-3-C5NH4)PO3}(4)](12-) (3). Poly-anions 1-3 were prepared in a one-pot reaction of the dinuclear, dicationic {(Mo2O4-)-O-V(H2O)(6)}(2+) with 1-hydroxo-2-(3-pyridyl)ethylidenediphosphonate (Risedronic acid) in aqueous solution. Polyanions 1 and 2 are mixed-valent Mo-VI/V species with open tetranuclear and hexanuclear structures, respectively, containing two diphosphonate groups. Polyanion 3 is a cyclic octanuclear structure based on four {(Mo2O4)-O-V(H2O)} units and four diphosphonates. Polyanions 1 and 2 crystallized as guanidinium salts [C(NH2)(3)](5)H[{(MoO3)-O-VI}(2){(Mo2O4)-O-V}-{HO3PC(O)(CH2-3-C5NH4)PO3} (2)]center dot 13H(2)O (1a) and [C(NH2)(3)](6)H-2[{(Mo2O6)-O-VI}(2)-{(Mo2O4)-O-V} {O3PC(O)(CH2-3-C5NH4)PO3}(2)]center dot 10H(2)O (2a), whereas polyanion 3 crystallized as a mixed sodium-guanidinium salt, Na-8[C(NH2)(3)](4)[{(Mo2O4)-O-V(H2O)}(4){O3PC(O)(CH2-3-C5NH4)PO3}(4)]cen ter dot 8H(2)O (3a). The compounds were characterized in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric and elemental analyses. The formation of polyanions 1 and 3 is very sensitive to the pH value of the reaction solution, with exclusive formation of 1 above pH 7.4 and 3 below pH 6.6. Detailed solution studies by multinuclear NMR spectrometry were performed to study the equilibrium between these two compounds. Polyanion 2 was insoluble in all common solvents. Detailed computational studies on the solution phases of 1 and 3 indicated the stability of these polyanions in solution, in complete agreement with the experimental findings.