Oxide cathodes prepared on a Si‐Ni alloy base metal have an interface of barium orthosilicate. The thickness of this layer is measured by means of an x‐ray method and found to increase with the life of the cathode and to be of the order of 10−3cm. Measurements of the effective, specific electrical conductivity were made and compared with the conductivity of the coating, (BaSr)O. Both materials exhibit a conductivity‐temperature variation characteristic of semiconductors; however, the conductivity of the interface was always less than that of the coating. The interface layer influences the thermionic emission characteristics of the cathode due to an interface voltage developed by the flow of emission current. A retarding‐potential method is developed for determining this voltage.