By the use of radioactive tracer techniques it is shown that the strontium evolution from an oxide coated cathode provides a measure of the rate of reduction of the coating. For the cases of carbon, aluminum, manganese, and tungsten the results indicate that the rate of reduction of the coating is controlled by diffusion of the reducing agents from the base nickel. The rate at which common reducing agents (excluding carbon) reach the nickel‐coating interface is proportional tot−½for lifetimes up to 104hours. For oxide coated cathodes operated without current drain the thermionic emission as measured by pulse means is observed to correlate with the rate of reduction of the coating. The development of thermionic activity can be understood in terms of a model in which the concentration of donors in the oxide particles is controlled by the surface concentration of the alkaline earth metals. This surface concentration is controlled by the rate of reduction of the coating, with the further postulate that there is a maximum effective surface concentration.