The thermodynamic selectivity for Gd3+ relative to Ca2+, Zn2+, and Fe3+ of two ligands of potential interest as magnetic resonance imaging (MRI) contrast agents has been determined by NMR spectroscopy and potentiometric and spectrophotometric titration. The two hexadentate ligands TREN-6-Me-3,2-HOPO (H(3)L2) and TREN-bisHOPOT-AM-EA (H(4)L3) incorporate 2,3-dihydroxypyridonate and 2,3-dihydroxyterephthalamide moieties. They were chosen to span a range of basicity while maintaining a structural motif similar to that of the parent ligand, TREN-1-Me-3,2-HOPO (H(3)L1), in order to investigate the effect of the ligand basicity on its selectivity. The 1:1 stability constants (beta(110)) at 25 degreesC and 0.1 M KCl are as follows. L2: Gd3+, 20.3; Ca2+, 7.4; Zn2+, 11.9; Fe3+, 27.9. L3: Gd3+, 24.3; Ca2+, 5.2; Zn2+, 14.6; Fe3+, 35.1. At physiological pH, the selectivity of the ligand for Gd3+ over Ca2+ increases with the basicity of the ligand and decreases for Gd3+ over Fe3+. These trends are consistent with the relative acidities of the various metal ions;- more basic ligands favor harder metals with a higher charge-to-radius ratio. The stabilities of the Zn2+ complexes do not correlate with basicity and are thought to be more influenced by geometric factors. The selectivities of these ligands are superior to those of the octadentate poly(aminocarboxylate) ligands that are currently used as MRI contrast agents in diagnostic medicine.