Mixtures of paramagnetic Gd@C82(OH)X and diamagnetic C82(OH)X (X ∼ 30) fullerenols in water have been studied by polarized neutrons at the concentrations C = 0.04 - 2 % wt. At ambient temperature the concentration increase and magnetic field B = 0.001 - 1.0 Tesla strengthening has initiated a growth of molecular correlations at the scale RC ~15-20 nm, when the aggregation number achieved very high magnitudes ∼ 5•104 while rather small contribution of nuclear-magnetic interference (≤ 0.1 %) to the total scattering cross section was detected. Thus, in these systems a molecular self-assembly has dominated. This molecular ordering under magnetic field seems to be crucial for their ability to provide a high contrast in medical MRI-diagnostics.