Ozone, O₃, forms more rapidly when one of its end O atoms is ¹⁷O or ¹⁸O instead of the normal isotope, ¹⁶O. Quantum chemists refer to this phenomenon as the ‘h effect’. This mass-independent fractionation pattern, denoted as D¹⁷O, is distinct from the normal mass-dependent fractionation that arises because of the difference in mass between the three isotopes. The positive D¹⁷O signal is passed on to CO₂ via O₃ photolysis and production of O(¹D), followed by isotopic exchange between O(¹D) and CO₂. Residual atmospheric O₂ develops negative D¹⁷O because of mass balance. Today, the atmospheric O₂:CO₂ ratio is large, so the D¹⁷O signal in O₂ is small. But in the Mid-Proterozoic eon (1.8-0.8 b.y. ago), CO₂ levels were higher and O₂ was lower, so D¹⁷O in O₂ was large enough to be recorded in sulfate minerals produced from oxidation of pyrite, FeS₂. Implications of these data for Mid-Proterozoic atmospheric O₂ levels will be discussed.