Diffusion rates of tungsten in single crystal and polycrystalline nickel were measured at temperatures between 1100°C and 1275°C by using radioactive tungsten185and a sectioning technique. Grain boundary diffusion was observed by use of radioautographs. A simple vacancy mechanism accounts for nearly all of the diffusion; some refinement is required for the transport observed at greater depths.The rate of reaction of polycrystalline 4.9% tungsten‐nickel alloy with SrO was measured by the hydrogen evolution technique at temperatures between 830°C and 1280°C. The rate indicates that the production of Sr in amounts up to 200 &mgr;g cm−2is not limited by diffusion of tungsten in the nickel core.