The minor constituents of toluene extracts of three fullerene-rich materials have been characterized by on-line LC–MS techniques, incorporating both positive and negative ion mass spectra obtained by atmospheric pressure chemical ionization. Two of the materials were soots, produced by resistive heating of graphite in an inert atmosphere, from different commercial suppliers. The third material was obtained as a condensate from a controlled laminar flame, and was shown previously (Anacleto etal:, Rapid Commun. Mass Spectrom. 6, 214 (1992)) to contain significant quantities of compounds that behaved under the present LC–MS conditions as isomers of the C60 and C70 fullerenes, and which reverted to the latter upon heating. This finding was confirmed here, and extended to the higher clusters C76, C84, C90, and C94. One of the graphite-derived soots contained monoxides of the carbon clusters as the principal minor components, while the other soot contained hydrogenated species including C60H2, C60H4, C70H2, and (C60•CH4). The flame-generated material contained all of these minor constituents, together with complexes of C60 with larger aliphatic molecules as well as large quantities of polycyclic aromatic hydrocarbons and related species. Photo-oxidation of a purified C60 preparation, from the graphitic soot containing mostly monoxide impurities, was shown to lead to increased levels of the mono-, di-, and tri-oxides of C60.