Stratospheric CO2 is found to be enriched in heavy oxygen isotopes relative to tropospheric CO2 values caused by photo-induced isotopic exchange with stratospheric ozone, a gas highly enriched in heavy isotopes relative to the ambient oxygen. The transfer of isotopic enrichment from the ozone pool to the CO2 pool takes place through CO*3 formed by reaction of CO2 with O(1D) derived from O3 photolysis. Recent stratospheric measurements indicate that the transfer process results in CO2 getting more enriched in 17O compared to 18O. Experiments, reported here, investigate this exchange and determine the effects of the initial isotopic composition of CO2 and O3 on the final isotopic composition of the exchanged CO2. It is seen that the slope relating 17O and 18O enrichments in CO2 depends on the initial isotopic composition of the two gases. Interestingly, with CO2 of tropospheric composition and O3 of stratospheric composition the observed slope (1.82 ¿ 0.04) is close to the value (1.71) found for stratospheric CO2. The apparent extra 17O enrichment in CO2 resulting in slope higher than that of O3 can be explained by simple mixing if one assumes that O(1D) originates mainly from the asymmetric ozone species and if additional fractionation occurs because of differences in collision frequencies of isotopic species and possibly also during dissociation of CO*3 to CO2 and O atom.