We report here the physical-chemical characterization of a highly water-soluble Gd(III) complex (complex 1) formed with the asymmetric hexadentate ligand H(4)bedik (2,2-((2-(bis (carboxymethyl)amino)benzyl)azanediyl)diacetic acid). The number of water molecules directly coordinated to the metal, q = 2.2, was assessed by measuring luminescence lifetimes of the Tb(III) analogue (complex 2) in H2O and D2O and it further was confirmed by O-17 chemical shift measurements on the Gd(III) complex. The complex has moderate thermodynamic stability and remains insensitive to physiological anions [biphosphate (HPO42-), bicarbonate (HCO3-)] and pH variation (in the range 5-10) of the medium as evidenced by negligible changes in longitudinal relaxivity (r(1)) at 1.41 T, 25 degrees C. The Gd(III) complex exhibits very fast water exchange, among the fastest reported for Gd(III) chelates, and high relaxivity at high magnetic fields (r(1) = 7.47mM(-1)s(-1) at 9.4 T, pH similar to 7 and 25 degrees C). Interestingly, the very high positive value of the activation entropy indicates a dissociatively activated water exchange for this eight-coordinate complex.