The reaction of Gd(III) with asymmetric tetramine 1,4,7,11-tetraazaundecane (2,2,3-tet, L(1)) ligand has been studied via NMR spectroscopy. The ligand proton longitudinal relaxation rates (R(1)) have been used to estimate the distances of these protons from the Gd(III) center, in Gd(III)-L(1) reaction solutions, in H(2)O/D(2)O 5/1 mixtures. Two Gd(III) complexes [Gd(III)(L(1))(NH(3))(H(2)O)(4)](CH(3)COO)(3)*2H(2)O (1) and [Gd(III)(L(1))(NH(3))(H(2)O)(2)]Cl(3)*EtOH (2) have been isolated and characterized by elemental analyses, TGA, IR, NMR and relaxometry measurements. The NMR relaxation measurements of 2 in aqueous solutions have been performed, under various temperature or concentration conditions, and compared with those of the commercial contrast agents Gd(III)-DTPA and Gd(III)-DTPA-BMA. It has also been studied the influence of (i) the Gd(III) inner-sphere water molecule number (q) alteration and (ii) the steric constraint enhancement on the metal site, over the relaxation rate values of the parent aqueous solution of Gd(III)-2,2,3-tet, and of the aqueous solutions of 2.