Stable aqueous fullerene colloidal suspensions (nC60) are demonstrated to rely on the [6,6]-closed epoxide derivative of the fullerene (C60O) for stability. This derivative is present, though often unrecognized, in small quantities in nearly all C60 starting materials due to a reaction with air. The low-yield formation of nC60 from organic solvent solutions results from a preferential partitioning and thus enrichment of C60O in the colloidal particles. This partitioning is significantly retarded in the nC60 synthesis method that does not involve organic solvent solutions: long-term stirring in water. Instead, this method relies on trace levels of ozone in the ambient atmosphere to produce sufficient C60O at the surfaces of the nC60 particles to allow stable suspension in water. Controlled-atmosphere syntheses, deliberate C60O enrichment, light scattering measurements, and extraction followed by HPLC analysis and UV–visible absorption spectroscopy support the above model of nC60 formation and stabilization.