Medium sized molecules endowed with multiple Gd3+ complexes are efficient high magnetic field MRI contrast agents. The novel ligand Mes(DO3A)3, presenting three DO3A (1,4,7,10-tetraazacyclododecane-N,N′,N′′-triacetatic acid) units grafted on the methyl positions of a central mesitylene (1,3,5-trimethylbenzene), has been synthesized. Designed as an MRI contrast agent, this ligand is complexed with Gd3+ and its efficiency is characterized by variable field 1H-NMR and variable temperature 17O-NMR. The evaluation of the relaxation and paramagnetic chemical shift data allowed the identification of an undesired binuclear complex which is obtained by using the classical procedure for complexation as described in the literature. An intramolecular capping mechanism appears to be responsible for the failure to introduce a third Gd3+ ion into the ligand. A new alternative method, based on pre-complexation with Mg2+ followed by transmetallation is described here and leads to the expected trinuclear Gd3+ complex [Mes{Gd(DO3A)(H2O)2}3]. The rate constants for the water exchange (kex298 = 32 × 106 s−1) for the bi- and trinuclear complex appeared to be the same, which is surprising in relation to the difference in the charge of the complex and to the difference in the number of coordinated water molecules, one and two per Gd3+ for the binuclear and trinuclear complex, respectively.