A new potentially multidentate hexaprotic ligand H-6[TETA-(PO)(2)] has been prepared by reaction of ethylenediamine-N,N'-diacetic acid (EDDA), paraformaldehyde, and phosphinic acid; its coordination properties with three lanthanide ions (La3+, Gd3+, and Lu3+) have been explored. The structures of the complexes were studied in aqueous solution by potentiometric pH titrations and by P-31 NMR spectroscopy. Four acidity constants were determined potentiometrically in the range 2.5 < pH < 14. The four measured pK(a) values can be divided into two groups, and within each group the initial deprotonation was found to have little effect on the second. Variable temperature 31P and P-31{H-1} EXSY NMR spectra showed that, for [Lu(TETA-(PO)(2))](3-), the two phosphorus atoms exist in different chemical environments and undergo an exchange process which is very fast on the NMR time scale at room temperature. This result is consistent with one of the phosphinate residues coordinating the metal ion and exchanging with a free analogue. In the case of [La(TETA-(PO)(2))](3-), only one temperature invariant signal is observed in P-31 NMR spectra; it corresponds to both phosphinate residues remaining uncoordinated to La3+. The stability of [Ln-(TETA-(PO)(2))](3-) has an order of La3+ > Gd3+ > Lu3+. The coordination of one phosphinate residue to Lu3+ brings the metal ion closer to the plane of four nitrogens and farther from the four carboxylate arms, resulting in [Lu(TETA-(PO)(2))](3-) having a lower stability than the corresponding La3+ and Gd3+ complexes. A pM-pH distribution diagram showed that introducing two phosphinate groups into TETA renders [Gd(TETA-(PO)(2))](3-) more stable than [Gd(TETA)](-). The selectivity factor of the ligand for Gd3+ vs Ca2+, Zn2+, and Cu2+ has been calculated, and the hydration number for [Dy(TETA-(PO)(2))](3-) has been measured by O-17 NMR spectroscopy to be zero.