The amonabactins are a series of four bis(catecholate) siderophores isolated from the pathogenic organism, Aeromonas hydrophila. As tetradentate ligands, they cannot singly satisfy the octahedral coordination sphere of iron. The solution coordination chemistry of the amonabactins has been elucidated using potentiometric and spectrophotometric titrations, circular dichroism, and mass spectroscopy. They form 2:3 metal:ligand complexes at high pH and excess Ligand. Their complexation behavior is essentially identical to one another, with log beta(230) = 86.3. At lower pH, they preferentially form a 1:1 bis(catecholato)bis(aqua) iron(III) species, with log beta(110) = 34.3. The 2:3 complexes show a very slight Delta preference in chirality at the metal center, while the 1:1 complexes are achiral. The biological implications of these properties are discussed.