We report the synthesis and characterization of the macrocyclic ligand 2,2'- ((2- (3,9-bis(carboxymethyl)-3,6,9-triaza-1 (2,6) -pyridinacyclodecaphane-6-yl)-ethyl)-azanediyl)diacetic acid (H4L) and several of its complexes with lanthanide ions. The structure of the free ligand was determined using X-ray diffraction measurements. Two N atoms of the pyclen moiety in the trans position are protonated in the solid state, together with the exocyclic N atom and one of the carboxylate groups of the ligand. The relaxivity of the Gd3+ complex was found to increase from 6.7 mM(-1) s(-1) at pH 8.6 to 8.5 mM(-1) s(-1) below pH approximate to 6.0. Luminescence lifetime measurements recorded from H2O and D2O solutions of the Eu3+ complex evidence the presence of a single complex species in solution at low pH (similar to 5.0) that contains two inner-sphere water molecules. DFT calculations suggest that the coordination environment of the Ln(3+) ion is fulfilled by the four N atoms of the pyclen unit, two oxygen atoms of the macrocyclic acetate groups, and an oxygen atom of an exocyclic carboxylate group. The two inner-sphere water molecules complete coordination number nine around the metal ion. At high pH (similar to 9.3), the lifetime of the excited D-5(0) level of Eu3+ displays a biexponential behavior that can be attributed to the presence of two species in solution with hydration numbers of q = 0 and q = 1. The H-1 NMR and DOSY spectra recorded from solutions of the Eu3+ and Y3+ complexes reveal a structural change triggered by pH and the formation of small aggregates at high pH values.