A new cryogenic collection system has been flown on board a balloon gondola to obtain separate samples of ozone and carbon dioxide without entrapping major atmospheric gases. Precision laboratory isotopic analysis of CO2 samples collected between 26 and 35.5 km show a mass-independent enrichment in both 17O and 18O of about 11 per mil (%) above tropospheric values. Ozone enrichment in its heavy isotopes was 9 to 16% in 50O3 and 8 to 11% in 49O3, respectively [Schueler et al., 1990>. A mechanism to explain the isotope enrichment in CO2 has been recently proposed by Yung et al. [1991>. Their model is based on the isotope exchange between CO2 and O3 via O(1D), resulting in a transfer of the ozone isotope enrichment to carbon dioxide. Predicted enrichments and measured values agree well. ¿American Geophysical Union 1991