Dissociation of ozone in the Chappuis bands has been used as an O atom source to study isotope effects occurring in the O(3P)+O2(2&Sgr;g) recombination reaction. The ozone produced was found to be enriched in both of its heavy isotopes. The pressure dependence (5--1000 torr) and temperature dependence (127--360 K) of this isotope effect have been investigated. The enrichment is approximately constant from 5 torr to 100 torr and decreases at higher pressures. It increases with temperature, with 50O3 showing a slightly faster rate of increase than 49O3. The results of this experiment have clearly isolated the source of the isotope effect to the gas phase O(3P)+O2(3&Sgr;g) recombination reaction. For comparison, we also present isotope ratios of ozone formed in an electric discharge. None of the results, however, have shown the large enhancement of 40% or more in mass 50 observed in some stratospheric measurements. ¿ American Geophysical Union 1990