Endometallofullerenols Fe@C60(OH)30 were synthesized by a new procedure using a special molecular precursor in the course of electric arc evaporation of a composite carbon electrode. Aqueous solutions of fullerenols Fe@C60(OH)30 were studied by small-angle neutron scattering (SANS) and viscometry under the conditions of transition from dilute to concentrated systems in two media: acidic (pH 3.7) and weakly alkaline (pH 7.8). In a weakly alkaline medium, fullerenols form relatively dense clusters (correlation radius RC ~ 17 nm), and in acidic medium, less dense structures of size RC ~ 18 nm in the concentration interval с = 0.5–1.4 wt %. The mechanisms of aggregation of fullerenol molecules in aqueous solutions are discussed. The water-soluble derivatives Fe@C60(OH)30 obtained show promise for biomedical applications in magnetic resonance imaging as contrasting agents that are potentially less toxic than presently used contrasts based on gadolinium chelate complexes.