An analysis of the bulk diffusion properties of the stage‐III defect is given utilizing the experimental results presented in Part I. It is found that the motion energy of the defect is 0.64±0.03 eV and, in addition, it is shown that this is a true motion energy and not a defect‐impurity‐atom binding energy. After consideration of the extremely small lattice‐defect concentrations used (≤10−12), available information concerning single‐vacancy motion energies, and the simple‐defect spectrum produced by the &ggr; irradiation, it is concluded that the stage‐III defect can only be identified as a freely migrating interstitial atom.