Two products, A and B, with the formula C120O2 have been characterized by HPLC, MALDI-TOF MS, FT-Raman, and C-13 NMR spectroscopy. C120O2(A) was found as a byproduct accompanying C120O in the solid-phase reaction at 200 degrees C of mixtures containing approximately a 4:1 ratio of C-60 to C60O and traces of C60O2 Raman spectra indicated that A has a furanoid bridge (analogous to C120O) linking a C-60 fullerene cage with a C60O cage containing an epoxy group. C-13 : NMR confirmed this and showed that the isolated product A consists of two closely related isomers with C-1 symmetry in a 4:1 ratio. C120O2(A) can be formed by the [3 + 2] cycloaddition of C60O to a second C60O or possibly by the addition of C60O2 to C-60 The 16 possible regioisomers of A were modeled by semiempirical methods, and isomers with the epoxy group located in the cis-l, e, or trans positions relative to the furanoid bridge have lower energies compared with those of cis-2 or -3 forms. The NMR and modeling data provide strong evidence that the two isomers of A represent the two possible cis-l configurations. The other isomers (but not cis-l forms) can react further with C-60 to form trimeric C180O2 compounds, which were also observed. Thermolysis of solid C120O at temperatures above 300 degrees C yielded C120O2(B) and other products but no A. C120O2(B) consists Of a Single isomer with C-2v symmetry in which the C-60 cages are linked by two neighboring furanoid bridges in a cis-l configuration. Modeling calculations indicate that this isomer has an energy that is significantly lower than that of other possible double-bridged isomers and ca. 50 kcal/mol below the energy of the A isomers.