The formation kinetics of ozone has been studied using isotopes and pulse radiolysis combined with time-resolved UV absorption spectroscopy. An enhancement of (9.8¿2.6)% was found for the rate constant for the reaction of 18O with 18O18O relative to that for the 16O+16O16O reaction. The average formation rate for unsymmetric 18O16O16O and 16O18O18O from O2 and 18O respectively was enhanced by (14.7¿2.8)%. For the formation of a mixture of symmetric and unsymmetric ozone species from O18O the enhancement was (3.6¿2.4)%. This leads to the conclusion that both mass and symmetry affect the rate constant for formation of isotopic ozone. The results are compared with recent enhancement studies from the literature, and an apparent conflict is discussed. ¿ American Geophysical Union 1995.