Two novel ligands containing pyridine units and phosphonate pendant arms, with ethane-1,2-diamine (L2) or cyclohexane-1,2-diamine (L3) backbones, have been synthesized for Ln complexation. The hydration numbers obtained from luminescence lifetime measurements in aqueous solutions of the EuIII and TbIII complexes are q = 0.6 (EuL2), 0.7 (TbL2), 0.8 (EuL3), and 0.4 (TbL3). To further assess the hydration equilibrium, we have performed a variable-temperature and -pressure UV-vis spectrophotometric study on the EuIII complexes. The reaction enthalpy, entropy, and volume for the hydration equilibrium EuL EuL(H2O) were calculated to be H = -(11.6 ± 2) kJ mol-1, S = -(34.2 ± 5) J mol-1 K-1, and = 1.8 ± 0.3 for EuL2 and H = -(13.5 ± 1) kJ mol-1, S = -(41 ± 4) J mol-1 K-1, and = 1.7 ± 0.3 for EuL3, respectively. We have carried out variable-temperature 17O NMR and nuclear magnetic relaxation dispersion (NMRD) measurements on the GdL2(H2O)q and GdL3(H2O)q systems. Given the presence of phosphonate groups in the ligand backbone, a second-sphere relaxation mechanism has been included for the analysis of the longitudinal 17O and 1H NMR relaxation rates. The water exchange rate on GdL2(H2O)q, = (7.0 ± 0.8) × 108 s-1, is extremely high and comparable to that on the GdIII aqua ion, while it is slightly reduced for GdL3(H2O)q, = (1.5 ± 0.1) × 108 s-1. This fast exchange can be rationalized in terms of a very flexible inner coordination sphere, which is slightly rigidified for L3 by the introduction of the cyclohexyl group on the amine backbone. The water exchange proceeds via a dissociative interchange mechanism, evidenced by the positive activation volumes obtained from variable-pressure 17O NMR for both GdL2(H2O)q and GdL3(H2O)q (V = +8.3 ± 1.0 and 8.7 ± 1.0 cm3 mol-1, respectively).