Abstract. Stratospheric and upper tropospheric air samples were collected during 1994–2004 over Sanriku, Japan and in 1997 over Kiruna, Sweden. Using these archived air samples, we determined the triple oxygen-isotope composition of stratospheric CO₂ and the N₂O mixing ratio. The maximum Δ¹⁷O_CO2 value of +12.2‰, resembling that observed previously in the mesosphere at 60 km height, was found in the middle stratosphere over Kiruna at 25.6 km height, suggesting that upper stratospheric and mesospheric air descended to the middle stratosphere through strong downward advection. A least-squares regression analysis of our observations on a δ¹⁸O_CO2−δ¹⁷O_CO2 plot (r²>0.95) shows a slope of 1.63±pm0.10, which is similar to the reported value of 1.71±0.06, thereby confirming the linearity of three isotope correlation with the slope of 1.6–1.7 in the mid-latitude lower and middle stratosphere. The slope decrease with increasing altitude and a curvy trend in three-isotope correlation reported from previous studies were not statistically significant. Using negative linear correlations of Δ¹⁷O_CO2 and δ¹⁸O_CO2 with the N₂O mixing ratio, we quantified triple oxygen-isotope fluxes of CO₂ to the troposphere as +48‰ GtC/yr (Δ¹⁷O_CO2) and +38‰ GtC/yr (δ¹⁸O_CO2) with ~30% uncertainty. Comparing recent model results and observations, underestimation of the three isotope slope and the maximum Δ¹⁷O_CO2 value in the model were clarified, suggesting a smaller O₂ photolysis contribution than that of the model. Simultaneous observations of δ¹⁸O_CO2, δ¹⁷O_CO2, and N₂O mixing ratios can elucidate triple oxygen isotopes in CO₂ and clarify complex interactions among physical, chemical, and photochemical processes occurring in the middle atmosphere.