The new antimonato polyoxovanadate {Zn(en)₃}₃[V₁₅Sb₆O₄₂(H₂O)]•3en•10H₂O (en = ethylenediamine) synthesized under hydrothermal conditions exhibits a remarkable solubility in water. Electrospray ionization mass spectrometry (ESI MS) investigations on an aqueous solution demonstrate that the cluster core is fully intact for 72 h. At longer times a slow transformation into a {V₁₄Sb₈O₄₂} cluster is observed. The conversion reaches 50% after 14 days and is complete after approximately 20 days. The rate of the {V₁₅Sb₆} → {V₁₄Sb₈} cluster transformation significantly increases, when ammonium acetate is added. Applying the new compound as a synthon in the presence of 1,10-phenanthroline (phen) afforded crystallization of {Zn(phen)₃}₂[Zn(en)₂V₁₅Sb₆O₄₂(H₂O)]•23H₂O after short reaction times, while addition of Sb₂O₃ led to fast crystallization of {(Zn(en)₂(H₂O)₂)(Zn(en)₂)}[Zn(en)₂V₁₅Sb₆O₄₂(H₂O)]•8.5H₂O. In the crystal structure of {Zn(en)₃}₃[V₁₅Sb₆O₄₂(H₂O)]•3en•10H₂O, the en molecules are attached to the cluster anion by Sb-N bonds. In the structures of the resulting two new compounds, the [V₁₅Sb₆O₄₂(H₂O)]^6- anions are expanded by Zn²⁺-centered complexes through Zn-O-V bond formation.