Diffusion of ion implanted Be in AlxGa1−xAs was studied as a function of Al concentration and annealing temperature and was compared to diffusion in GaAs. The behavior of Be in AlxGa1−xAs is similar to that in GaAs, even showing the anomalous behavior of increasing redistribution with decreasing temperature. The diffusivity of Be appears to increase with Al content which may be due to increasing bonding strength of matrix atoms with the addition of Al, preventing the easy transferral of Be from interstitial to substitutional sites. The oversaturation of Be interstitials may also explain the persistence of anomalous diffusion behavior in AlxGa1−xAs with respect to anneal temperature. This is discussed in terms of the substitutional‐interstitial diffusion mechanism, the relative amount of interstitial and substitutional Be, and the relative difficulty of moving from an interstitial to a substitutional site. ©1995 American Institute of Physics.