The use of the O- 18(p,n)F- 18 nuclear reaction to study the mechanism of oxidation of metals offers a number of advantages over conventional marker techniques. In dense, adherent scales, for example, not only can the diffusing species be identified unambiguously but, in systems in which oxygen is the mobile component, valuable information concerning the mode of transport may also be deduced from the shape of the O- 18 concentration profile. The utilization of O- 18 as a tracer which can be detected by autoradiographic methods through subsequent protonactivation provides the unique capability of determining whether the tranport of oxygen takes place by diffusion through the solid reaction product or by some other means.