Using small angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS), we have studied aqueous solutions of a fullerene-based star ionomer-hexa(sulfobutoxypentylcarboxy)[60]fullerene (FC10S). With six sulfobutoxypentyl arms randomly bounded on the C-60 cage, FC10S demonstrates a good solubility in water. The scattering results show that the water-soluble fullerene-based ionomers of a starlike molecular morphology form rodlike aggregates in water solutions. With the contrast provided by SAXS and SANS, we extract detailed structural information for the rodlike aggregates, including arm distribution, aggregation number similar to32, and the rod radius and length of 21 Angstrom and 160 Angstrom, respectively. We also extract an averaged ionization factor of similar to0.03 for the FC10S aggregates, using the structural factor derived from the mean spherical approximation model for the charge interacting aggregates. The aggregation structure changes little within the concentration range studied (0.5-4%). Compared to the aggregation characteristics of FC4S, a similar fullerene-based ionomer with six shorter sulfobutyl arms, FC10S, has a much higher aggregation tendency due to the longer sulfobutoxypentyl arms anchored on the C-60 cage.