We report observations of stratospheric CO ₂ that reveal surprisingly large anomalous enrichments in ¹⁷ O that vary systematically with latitude, altitude, and season. The triple isotope slopes reached 1.95 ± 0.05(1σ) in the middle stratosphere and 2.22 ± 0.07 in the Arctic vortex versus 1.71 ± 0.03 from previous observations and a remarkable factor of 4 larger than the mass-dependent value of 0.52. Kinetics modeling of laboratory measurements of photochemical ozone–CO ₂ isotope exchange demonstrates that non–mass-dependent isotope effects in ozone formation alone quantitatively account for the ¹⁷ O anomaly in CO ₂ in the laboratory, resolving long-standing discrepancies between models and laboratory measurements. Model sensitivities to hypothetical mass-dependent isotope effects in reactions involving O ₃ , O( ¹ D), or CO ₂ and to an empirically derived temperature dependence of the anomalous kinetic isotope effects in ozone formation then provide a conceptual framework for understanding the differences in the isotopic composition and the triple isotope slopes between the laboratory and the stratosphere and between different regions of the stratosphere. This understanding in turn provides a firmer foundation for the diverse biogeochemical and paleoclimate applications of ¹⁷ O anomalies in tropospheric CO ₂ , O ₂ , mineral sulfates, and fossil bones and teeth, which all derive from stratospheric CO ₂ .