We present model calculations to investigate the isotopic behavior of both heavy carbon dioxide molecules C17O16O and C18O16O in the stratosphere and mesosphere. For this purpose the existing theory of 18O transfer between O3 and CO2 via O(1D) and the electronically excited complex CO3*, as proposed by Yung et al. <1991>, is extended to 17O. Our one-dimensional model agrees well with atmospheric measurements performed till now, when reasonable assumptions for the isotopic composition of ozone in the middle atmosphere are made, and when in addition a mass-dependent fractionation process during the transfer of oxygen atoms is included. Best agreement between model results and atmospheric CO2 isotope data is obtained by assuming (1) an averaged O3 isotope composition in the middle atmosphere of Δ17O=(12.5¿1.5)% and Δ18O=(16¿1.5)% (versus Vienna standard mean ocean water) and (2) a mass-dependent isotope fractionation during the O atom transfer via CO3* of (32¿7)? for 17O and (64¿14)? for 18O, both relative to 16O. Effects of CO2 photolysis and its reformation via the reaction CO+OH→CO2+H on the CO2 isotope composition are found to be smalland may be neglected.¿ 1997 American Geophysical Union