The recently discovered [5,6]-open isomer of C60O has been found to undergo facile dimerization to form a new C-2 symmetry isomer of C120O2, which can be photodissociated with relatively high efficiency to regenerate monomeric [5,6]-C60O. High yield dimerization of [5,6]-C60O proceeds spontaneously in toluene solution near room temperature. On the basis of C-13 NMR spectroscopy, ab initio quantum computations, and HPLC retention patterns, the resulting C120O2 product has been deduced to be a nonpolar dimer of C-2 symmetry in which the C60O moieties are linked by two single bonds between sp(3)-hybridized carbon atoms adjacent to oxygen atoms. Photophysical properties of this dimer have also been measured and compared to those of C-120, the [2 + 2]-dimer of C-60. The ground-state absorption spectrum of C120O2 in toluene is slightly red-shifted relative to that of C-120, with a distinctive peak at 329 nm and an S-1-S-0 origin band at 704 nm. Its fluorescence spectrum shows two major peaks at 718 and 793 nm. In room-temperature toluene, the measured triplet state intrinsic lifetime of this C120O2 isomer is 34 +/- 2 mus, a value somewhat shorter than that of C-120 (44 mus). C120O2 undergoes photodissociation from its triplet state to regenerate monomeric [5,6]-C60O with quantum yields of 2.5% at 24 degreesC and 43% at 70 degreesC. It can therefore serve as a stable reactant for photolytic production of [5,6]-C60O. As a simple fullerene adduct that reacts under mild conditions, [5,6]-C60O may prove useful in special synthetic applications. Solutions of [5,6]-C60O are also unique because they can provide mixtures of a fullerene monomer and its dinner in a dynamic balance controllable by adjustment of concentration, temperature, and optical irradiation.