Diffusion of Si in GaAs is studied using implanted Si in undoped GaAs, implanted Si in Se‐, Si‐, and Zn‐doped GaAs, and grown‐in Si in epitaxial layer structures. No diffusion is observed in the undoped and Zn‐doped GaAs cases, a moderate level is observed in the Si‐doped case, and a significant amount is found for the Se‐doped and nonimplanted Si‐doped epitaxy cases. These results indicate that the diffusion is controlled by a Fermi level mechanism (probably via ionized gallium vacancies) and that implant damage inhibits diffusion by keeping the electron concentration and/or the ionized gallium vacancy concentration low.