We present records of the triple oxygen isotope signature (Δ(¹⁷O)) of atmospheric CO₂ obtained with laser absorption spectroscopy from two mid-latitude stations. Significant interannual variability is observed in both records. A model sensitivity study suggests that stratosphere–troposphere exchange, which carries high-Δ(¹⁷O) CO₂ from the stratosphere into the troposphere, causes most of the variability. This makes Δ(¹⁷O) a potential tracer for stratospheric intrusions into the troposphere.