The oxygen isotope anomaly in ozone was Ðrst identiÐed in isotopomers and with enrichments of 49O 3 50O 3 over 10%. Recent rate coefficient measurements imply a new interpretation of the anomaly. The large variability of the rate coefficients of ozone formation of about 50% characterizes the ozone O ] O 2 ] O 3 isotope e †ect and demonstrates its kinetic origin. Surprisingly, the rate coefficients correlate with the zero-point energy change of the oxygen molecules participating in the isotope exchange reactions but not with the mass of the ozone molecules. Rate coefficients of symmetric molecules depart from this linear relationship by about 20%.