Vacuum‐evaporated gold films were used as inert markers to study the oxidation mechanism of cobalt at 850°C. Gold‐marked and unmarked specimens were simultaneously oxidized for 138 hr. With the exception of the gold marker appearing as a dispersed row of spheriods within the CoO layer closest to the metal‐oxide interface, and the random distribution of voids in the specimens, the microstructures of the gold‐marked and unmarked specimens were identical, and are in agreement with microstructures reported in the literature.