The effect of atomic oxygen in the oxidation behaviour of some fourteen metals—Ni, Co, Fe, Cu, Ag, Au, Pd, Pt, Zr, Al, Ta, Sn, Zn and Cd—has been investigated. The reactions were followed gravimetrically, using a Sartorius microbalance; measurements were made over the temperature range 25–750°C. Atomic oxygen enhanced the rate of oxidation of metals which formed a single p-type oxide (e.g., Ni) but had little or no effect if the oxide scale was n-type, e.g., ZrO2, or composed of several oxides, e.g., FeO + Fe3O4+ Fe2O3. These results are explained in terms of Wagner's theory of oxidation, which is extended to include the effect of atoms in the gas phase. Atomic oxygen also increased the rate of reaction when the rate-limiting step was at the gas/oxide interface, e.g., in Cu oxidation, and with silver it caused extensive oxidation where none was detected using molecular oxygen.