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A Buckminsterfullerene/polyisoprene (C₆₀/PI) composite was synthesized at high-temperature, high-pressure (HP&HT) conditions. The composite has significantly improved tensile strength and Young’s modulus, by up to 49% and 88% per wt % C₆₀, respectively, which is much higher than for corresponding composites with carbon nanotube (CNT) fillers. The reinforcing action of C₆₀ fillers is different from that of CNTs as C₆₀ becomes covalently bonded to PI chains, and C₆₀ clusters in PI form C₆₀–C₆₀ covalent bonds. The latter are reversible and break by heating at 1 bar, which suggests improved recyclability of the material and indicates that carbon nanostructures can be used as strong reversible cross-linkers (“vulcanizers”) in elastomers.