Gd-Fullerenols (Gd@C82(OH)40) are water-soluble Gd endohedral metallofullerenes used with in vivo MRI (8). The encapsulation of Gd has three electrons transferred to the C82 fullerene cage, resulting in zwitterions [Gd3+@C82(OH)403-] in which seven unpaired electrons are located on the 4f-orbital of gadolinium ion and one unpaired electron is located on the fullerene cage. These unpaired electrons contribute to the paramagnetic moment of Gd@C82(OH)40. Nevertheless, the encapsulation of the Gd ion inside the carbon cage prevents direct coordination and exchange of water to Gd3+. The relaxation enhancement of Gd@C82(OH)40 is therefore quite different from the classic inner-sphere mechanism in conventional Gd chelates such as Gd-DTPA but depends on its paramagnetic moment and intermolecular aggregation effect (1). The aggregation leads to an increase in the rotational correlation time and alterations in the water exchange kinetics, which both influence the resultant relaxivity.