The nuclear magnetic resonance (NMR)-relaxation characteristics of protons in pure aqueous and buffer solutions of highly hydroxylated endofullerenols with paramagnetic gadolinium ions at ambient temperature have been studied. These systems were thoroughly characterized by SANS and examined by NMR at various resonance frequencies: f = 2 kHz–500 MHz. In all the cases, we have discovered much higher longitudinal and transversal relaxation rates as compared to the reference salt solutions with Gd⁺³ ions in the same range of concentrations (0.1–10.0 mM/l). We suppose that this effect is due to the fact that the objects studied reveal well-manifested abilities to the self-assembly in acidic conditions. As a result, the transversal relaxation rate (1/T₂) increases greatly, and also the longitudinal relaxation rate (1/T₁) demonstrates a broad maximum at resonant frequencies f ~ 20–100 MHz that are determined by the time of fullerenol diffusive rotations. The relaxation rates, which increase linearly with fullerenol content (0.1–10.0 mM/l), testify the stable assembly. The studied features of fullerenol assembly and its strong influence on the proton relaxation make it possible to suppose good prospects of these metal–carbon structures for biomedical applications, in particular, as contrast agents in MRI.