Two ozone rate coefficient ratios have been measured in different bath gases that include noble gases, O-2/N-2 mixtures as well as CH4 and CO2. While absolute rate coefficients for ozone formation differ by a factor of four between Ar and CO2 the rate coefficient ratios for O-16 + (OO)-O-18-O-18/O-16 + (OO)-O-16-O-16 and O-18 + (OO)-O-16-O-16/O-18 + (OO)-O-18-O-18 remain the same in all bath gases, the first ratio being over 50% higher than the second. This suggests that the ozone isotope effect maybe discussed within the framework of the Lindemann-Hinshelwood mechanism but with isotope specific steps: The selectivity arises in the first collision between O + O-2 which produces the activated complex O-3* and does not come from the subsequent collisional deactivation ana stabilisation through the third body.