Thoriated tungsten wires have been so heat‐treated in gas as to develop variations in grain size, ranging in length from 4 &mgr; to 3 cm. These variations produced marked changes in the thermionic emissive characteristics, such as rates of deactivation and of activation and duration of emissive life at normal operating temperature. The heat of diffusion remained unaffected. It was concluded that intergranular diffusion of thorium through tungsten is relatively too rapid to be measured and that the observed phenomenon depends purely on intragranular diffusion. This conception is in accord with the existence of a critical grain size for the realization of a maximum emissive life. Experiments are also described to demonstrate the spreading of thorium atoms over the surface of a single crystal of tungsten and to manifest the effect of slight strains in the lattice upon activation.