Single- and double-C-60-end-capped poly(ethylene oxide)s (PEOs) were prepared by reacting azido-terminated PEO with C-60. The aggregation behavior of these polymers in THF and water was studied by gel permeation chromatography, static and dynamic laser light scattering, and transmission electron microscopy. The solvent polarity, the amount Of C-60, and the length of PEO segments significantly affect the conformation and the size distribution of aggregates or clusters. The aggregation number of single C-60-end-capped PEO exceeds 10(4), far larger than nonionic surfactants consisting of PEOs end-capped with paraffinic chains due to the stronger hydrophobic character of C-60. Single-C-60-end-capped PEO forms much larger aggregates than those of double-end-capped PEOs, possibly due to the relatively higher mobility of the former polymer. The PEO chain length of the double-C-60-end-capped PEO controls the aggregate conformation and particle size distribution. It is believed that large aggregation complex comprises several smaller identical aggregates in THF solutions.