The diffusion of Be in GaAs is studied in samples which are molecular beam epitaxy GaAs with grown‐in Be. The Be diffusion profiles of samples annealed under various conditions are obtained using secondary ion mass spectrometry. SUPREM‐IV.GS, a simulator for GaAs and Si processing technology, is used to compare the experimental results with our models and to extract parameters. The Be diffusion profiles show a kink feature and a time‐dependent Be diffusivity which are successfully simulated. The intrinsic Be diffusivity, the Ga interstitial diffusivity, and the equilibrium concentration of Ga interstitials, all as a function of temperature, are obtained from this study:D+1Be=0.17 exp(−3.39 eV/kBT) cm2 s−1,DI=6.4×10−5 exp(−1.28 eV/kBT) cm2 s−1, andCI*int=4.7×1028 exp(−3.25 eV/kBT) cm−3. The role of nonequilibrium Ga point defects in the anomalous Be diffusion behavior is addressed. ©1995 American Institute of Physics.