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The He, Ne and Ar isotopic composition of gases collected in 2009 and 2012 from 13 production wells, injection wells and the CO₂ supply pipeline at the Cranfield CO₂-enhanced oil recovery field (MS, USA) have been measured in order to determine the extent to which they trace the fate of injected CO₂ in the reservoir. In the absence of samples of CO₂ pre-injection reservoir gas we use the Ne isotope composition of the production and injection well gases to determine the isotopic composition of the natural gas. The noble gas isotopes display binary mixing trends between the injected CO₂ and a CH₄-rich natural gas that is characterised by radiogenic He, Ne and Ar isotope ratios. ³He/⁴He and ⁴⁰Ar^*/⁴He ratios (where ⁴⁰Ar^* represents the non-atmospheric ⁴⁰Ar) display coherent relationships with CO₂ concentrations that can be used to trace and quantify the injected CO₂ in an engineered site over a sustained …