The gas phase bimolecular isotope exchange reaction between molecular oxygen and ozone has been investigated directly for the first time. Its rate coefficient is found to be less than 2¿10-25 cm3-sec-1 at 298 K, over six orders of magnitude below recent estimates. Much faster exchange was observed over condensed ozone at 77K, suggesting isotopic scrambling is catalyzed under these conditions. The low rate coefficient implies that homogeneous exchange between ground state oxygen and ozone molecules cannot play a significant role in heavy ozone chemistry. ¿ American Geophysical Union 1987