The concentration dependence of the diffusion coefficient of Zn in GaAs has been measured under excess arsenic vapor pressure at 850°C. The diffusion coefficient varied under these conditions only from 10−11to about 10−10cm2/sec as the zinc concentration increased from 1017to 1020zinc atoms/cm3. These results are compared with diffusion coefficients found in the absence of excess arsenic by the authors and other workers. The observed difference can be explained by means of an interstitial‐substitutional equilibrium. The excess arsenic increases the substitutional component of the diffusion and suppresses the interstitial component. These results are in good agreement with the model first suggested by Longini. The activation energy of the substitutional diffusion coefficient is found to be about 2.8 eV.