We propose a novel mechanism for isotopic exchange between CO2 and O3 via O(1D)+CO2 → CO3 * followed by CO3 * → CO2+O(3P). A one-dimensionaL model calculation shows that this mechanism can account for the enrichment in 18O in the stratospheric CO2 observed by Gamo et al. [1989>, using the heavy O3 profile observed by Mauersberger [1981>. The implications of this mechanism for other stratospheric species and as a source of isotopically heavy CO2 in the troposphere are briefly discussed. ¿ American Geophysical Union 1991