The dimeric C-70 oxides were synthesized from C-70/C70O mixed powder by hydrothermal treatment at 373 K for 12 h in aqueous NaOH solution. In this treatment, nucleophilic OH- ions are abundant and nucleophilic addition reactions were considered to synthesize dimeric fullerene oxides. The yield was estimated as ca. 2-3% summing up each isomer and also produced polyoxides, it is lower than that Of C120O. Three isomers Of C140O were isolated using High Performance Liquid Chromatography (HPLC) and characterized with UV- vis, FT-IR, and C-13 NMR and other methods. The results of these methods show a close similarity between C140O and C120O and support that our C140O samples exhibit the structure of conventional furan-bridged dimers. On the basis of the results of C-13 NMR and PM3 calculations, structure assignments for each sample were attempted.