The thermodynamic stabilities of the Gd III complexes of five hexadentate ligands which incorporate catecholate and hydroxypyridonate subunits with a range of basicities have been determined. The most stable Gd III complex at a physiological pH of 7.4 was observed for a ligand of intermediate basicity. The stability of this complex is comparable to that of Gd III complexes that feature eight-coordinate ligands and are currently used as magnetic resonance imaging contrast agents in diagnostic medicine. The accompanying bar chart shows pGd (= −log [Gd III free ]; total concentrations: [Gd III ] = 10 - 6 M, [L] = 10 - 5 M) for five hexadentate ligands which incorporate catecholate and hydroxypyridonate subunits at a physiological pH of 7.4. The basicity of the ligands increases in going from 1 to 5.