The chemistry of aluminum was explored in the presence of the physiological ligand citric acid and in low-pH aqueous media. As a result, the first dinuclear aluminum-citrate complex (NH4)(4)[Al-2-(C6H4O7)(C6H5O7)(2)].4H(2)O was isolated at low pH (similar to3.5), and was characterized by FT-IR spectroscopy and X-ray crystallography. The structural analysis reveals the presence of a dinuclear assembly of two aluminum ions octahedrally coordinated to three citrate ligands of differing protonation state. The NMR solution behavior of this complex emphasizes its time-dependent transformation into a number of variable nature species, ultimately leading to the thermodynamically stable trinuclear species. It also establishes the participation of the dinuclear complex as a viable component of the aqueous Al(III)-citrate speciation. The chemical and structural features of this novel low molecular mass species provide considerable insight into citrate's ability, as a natural ligand, to influence the chemistry of aluminum in a pH-dependent fashion, and potentially affect aluminum's (bio)distribution, absorption, accumulation, and biotoxicity at sensitive biological sites.