Water-soluble N,N-dimethyl[60]fulleropyrro- lidinium iodide (C60(C4H10N+)I-), when dissolved in binary mixtures of tetrahydrofuran and water, was found to exhibit remarkable solvatochromism. Correspondingly, the UV-vis spectra showed obvious changes by variation of the solvent composition of tetrahydrofuran and water. It was found that both concentration and temperature had strong influence on the stability of monomer-state C60(C4H10N+)I-, while they had little effect on the stability of aggregated C60(C4H10N+) I-. The formation of cluster was confirmed by TEM and AFM measurements.