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(3∑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, with50O3showing a slightly faster rate of increase than49O3. The results of this experiment have clearly isolated the source of the isotope effect to the gas phase O(3P) + O2(3∑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 measu