The equilibria and structure of complexes formed between the Ca²⁺ ion and the heptagluconate (Hglu⁻) ion in both neutral and alkaline solutions have been studied. In alkaline solutions an uncharged, multinuclear complex is formed with the composition of Ca₃Hglu₂(OH)₄ (or [Ca₃Hglu₂H₋₄]⁰) with an unexpectedly high stability constant (lg β_32–4 = 14.09). The formation of the trinuclear complex was deduced from potentiometry and confirmed by freezing-point depression measurements and conductometry as well. The binding sites of Hglu⁻ were determined from NMR measurements. Besides the carboxylate group, the O atoms on the second and third carbon atoms proved to be the most probable sites for Ca²⁺ binding.