Linear and symmetric star block copolymers of styrene and isoprene containing [C-60] fullerene were synthesized by anionic polymerization and appropriate linking postpolymerization chemistry. In all block copolymers, the C-60 was connected to the terminal polyisoprene (PI) block. The composition of the copolymers was kept constant (similar to 30% wt PI), whereas the molecular weight of the diblock chains was varied. The polymers were characterized with a number of techniques, including size exclusion chromatography, membrane osmometry, and H-1 NMR spectroscopy. The combined characterization results showed that the synthetic procedures followed led to well-defined materials. However, degradation of the fractionated star-shaped copolymers was observed after storage for 2 months at 4 degreesC, whereas the nonfractionated material was stable. To further elucidate the reasons for this degradation, we prepared and studied a four-arm star copolymer with the polystyrene part connected to C-60 and a six-arm star homopolymer of styrene. These polymers as well as linear copolymers end-capped, through -N <, with C-60 were stable. Possible reasons are discussed.