Abstract. Contrary to tropospheric CO₂ whose oxygen isotopic composition follows a standard mass dependent relationship, i.e. δ¹⁷O~0.5δ¹⁸O, stratospheric CO₂ is preferentially enriched in ¹⁷O, leading to a strikingly different relation: δ¹⁷O~1.7δ¹⁸O. It has been shown repeatedly that the isotope anomaly is inherited from O₃ via photolytically produced O(¹D) that undergoes isotope exchange with CO₂ and the anomaly may well serve as a tracer of stratospheric chemistry if details of the exchange mechanism are understood. We have studied the photochemical isotope equilibrium in UV-irradiated O₂-CO₂ and O₃-CO₂ mixtures to quantify the transfer of the anomaly from O₃ to CO₂ at room temperature. By following the time evolution of the oxygen isotopic compositions of CO₂ and O₂ under varying initial isotopic compositions of both, O₂/O₃ and CO₂, the isotope equilibria between the two reservoirs were determined. A very strong dependence of the isotope equilibrium on the O₂/CO₂-ratio was established. Equilibrium enrichments of ¹⁷O and ¹⁸O in CO₂ relative to O₂ diminish with increasing CO₂ content, and this reduction in the equilibrium enrichments does not follow a standard mass dependent relation. When molecular oxygen exceeds the amount of CO₂ by a factor of about 20, ¹⁷O and ¹⁸O in equilibrated CO₂ are enriched by (142±4)‰ and (146±4)‰, respectively, at room temperature and at a pressure of 225 hPa, independent of the initial isotopic compositions of CO₂ and O₂ or O₃. From these findings we derive a simple and general relation between the starting isotopic compositions and amounts of O₂ and CO₂ and the observed slope in a three oxygen isotope diagram. Predictions from this relation are compared with published laboratory and atmospheric data.