Microcrystalline mixture of fullerene dimer oxides (C120On,n=1,2,3,...) was extracted from decomposed C60Br24. Low-temperature FTIR studies of the product showed the absence of orientational ordering transitions indicating freezing of C60 rotations due to inter-cage bonding and epoxidation. Characteristic features due to inter-cage vibrations are seen in the far IR spectrum. Bond cleavages in MS/MS leave all the oxygen atoms of the dimer oxide with one fullerene molecule. We propose that dimers are formed by [3+2] cycloaddition of C60 with its oxides, C60On rather than cycloaddition of the monomeric oxides. Results are discussed based on the possible reaction sequences. © 2001 Elsevier Science B.V.