Complex formation and the coordination of zirconium with acetic acid were investigated with Zr K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) and single-crystal diffraction. Zr K-edge EXAFS spectra show that a stepwise increase of acetic acid in aqueous solution with 0.1 M Zr(IV) leads to a structural rearrangement from initial tetranuclear hydrolysis species [Zr-4(OH)(8)(OH2)(16)](8+) to a hexanuclear acetate species Zr-6(O)(4)(OH)(4)(CH3COO)(12). The solution species Zr-6(O)(4)(OH)(4)(CH3COO)(12) was preserved in crystals by slow evaporation of the aqueous solution. Single-crystal diffraction reveals an uncharged hexanuclear cluster in solid Zr-6(mu(3)-O)(4)(mu(3)-OH)(4)(CH3COO)(12).8.5H(2)O. EXAFS measurements show that the structures of the hexanuclear zirconium acetate cluster in solution and the solid state are identical.