Carbon dioxide in the middle atmosphere is mass independently enriched in the heavy oxygen isotopes relative to tropospheric values. That is, increasing with altitude, the 17O/16O ratio shows an additional enhancement over what is expected on the basis of the 18O/16O increase. As tropospheric CO2 has a mass-dependent isotopic composition that varies by less than 3? in Δ18O, isotopic measurements of middle atmospheric CO2, combined with a quantitative understanding of the enrichment mechanism, could provide valuable information regarding processes such as stratosphere-troposphere exchange and the mean age of an air mass. It is known that the mass-independent enrichment in stratospheric CO2 occurs when CO2 quenches an O(1D) atom formed by the photolysis of O3, but the details of this process remain uncertain. Here a series of laboratory and numerical experiments are presented which have been performed to study the time evolution and final equilibrium values of the CO2+O(1D) reaction in an effort to reach a better understanding of the CO2 enrichment mechanism. Results show that while the isotopic composition of the CO2 reservoir is qualitatively controlled by the isotopic composition of the O(1D) reservoir, there are a number of complicating factors. The simple mixing model discussed here consistently overpredicts the measured isotopic enrichment, thus indicating the CO2+O(1D) isotopic exchange is more complicated than has generally been recognized. ¿ 2000 American Geophysical Union