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Reversible anion binding in aqueous media at chiral Eu III and Tb III centers has been characterized by 1 H NMR and by changes in the emission intensity and circular polarization following direct or sensitized (365 nm) excitation via an alkylphenanthridinium chromophore. Using a series of heptadentate tri-amide or polycarboxylate ligands, the affinity for CO 3 2-/HCO 3 -, phosphate, lactate, citrate, acetate, and malonate at pH 7.4 was found to decrease as a function of the overall negative charge on the complex: citrate and malonate bound most strongly, and lactate and hydrogen carbonate also formed chelated ternary complexes in which displacement of both of the metal-bound water molecules occurred, which was confirmed by VT 17-O NMR measurements of the corresponding Gd complexes. The binding of carbonate was studied in particular, and 1 H NMR and CPL data were obtained that were consistent with the formation of a complex with a reduced helical twist about the metal center. Monohydrogen phosphate was bound in a monodentate manner, giving a monoaqua adduct. The binding of carbonate to cationic Eu complexes in the presence of a simulated extra-cellular anionic background at pH 7.4 was monitored by variation in the emission intensity, ratio of intensities (615/ 594 nm), and dissymmetry factors as a function of added total carbonate.