Dedicated to Professor Jean-Claude Bünzli on the occasion of his 65th birthday in friendshipThe protonation constants of 2-[4,7,10-tris(phosphonomethyl)-1,4,7,10-tetraazacyclododecan-1-yl]-acetic acid (H 7 DOA3P) and of the complexes [Ln(DOA3P)] 4À (Ln ¼ Ce, Pr, Sm, Eu, and Yb) have been determined by multinuclear NMR spectroscopy in the range pD 2 -13.8, without control of ionic strength. Seven out of eleven protonation steps were detected (pK H i ¼ 13. 66, 12.11, 7.19, 6.15, 5.77, 2.99, and 1.99), and the values found compare well with the ones recently determined by potentiometry for H 7 DOA3P, and for other related ligands. The overall basicity of H 7 DOA3P is higher than that of H 4 DOTA and trans-H 6 DO2A2P but lower than that of H 8 DOTP. Based on multinuclear-NMR spectroscopy, the protonation sequence for H 7 DOA3P was also tentatively assigned. Three protonation constants (pK MHL , pK MH2L , and pK MH3L ) were determined for the lanthanide complexes, and the values found are relatively high, although lower than the protonation constants of the related ligand (pK Introduction. -Acyclic and macrocyclic polyamino chelators bearing coordinating pendant arms have been largely investigated as ligands for lanthanide(III) (Ln III ) complexation, looking for compounds suitable for medical applications [1] [2]. For such applications, an ideal ligand is the one which forms lanthanide (Ln) complexes with high termodynamic stability and, even more imperative, with extremely high kinetic inertness. Indeed, a high thermodynamic stability does not necessarily prevent complex dissociation in vivo, even if a good selectivity for the lanthanide ion over other endogeneous metal ions (e.g. Ca