When a metal is oxidised in carbon dioxide, the growth of oxide film may be accompanied by the deposition of carbon which may return to the gas phase as carbon monoxide, remain in the oxide film or dissolve in the metal. Isotopic tracer techniques with carbon‐14 provide a simple method by which the amount of carbon deposition may be measured, and they also provide a unique method by which the fate of this carbon may be determined. The current experimental techniques are outlined and numerical values are given, with illustrative results, for the oxidation of beryllium, of a 20/25/niobium austenitic steel and of chromium.