An unstable intermediate has been detected and isolated in the reaction of ozone with C-60 in solution. On the basis of its UV-vis absorption spectrum, the measured release of O-2 in its decay to the epoxide C60O, and the first-order kinetics of this reaction, the intermediate has been identified as C60O3, a primary ozonide. This [6,6]-closed adduct of ozone with C-60 thermally dissociates to C60O plus O-2 in toluene solution, octane solution, and the solid phase with rate constants (at 23 degreesC) of 4.6 x 10(-2), 1.3 x 10(-3): and 3.0 x 10(-3) min(-1), respectively. The activation energy for dissociation in toluene solution is approximately 89 kJ mol(-1) Semiempirical quantum calculations indicate that the formation and subsequent dissociation of C60O3 are both exothermic processes. These findings constitute the first observation of a fullerene ozonide.