It has previously been shown that a 18O, 17O mass independent isotopic enrichment accompanies O3 formation. Precise definition of the mechanism has proven difficult and further experimental characterization of the mediating parameters is needed. The potential role of activated complex lifetime is determined by measuring the effect of pressure in the region where third body kinetic fall-off is known to occur (~4 atmospheres). In the pressure region ~8 to 45 atmospheres, the 18O, 17O enrichment decreases by nearly 84%O. For pressures between 45 to 66 atmospheres (1.6 ¿1021 molecules cm-3), the enhancement decreases by only 6%O and is absent (Δ18O=O) at 56 atmospheres. For pressures 56 to 87 atm, the product ozone is depleted in 18O in a nearly mass dependent fashion. The decrease in O3-18O enrichment with pressure initiates at a significantly lower pressure than expected, based upon the known reaction order pressure dependency. At present, no theoretical model quantitatively accounts for the observations. ¿ American Geophysical Union 1990