Experiments have been conducted to study the Si outdiffusion behavior in GaAs using predoped samples. The results showed that the Si diffusivity values are dependent on the As4 vapor-phase pressure in the ambient, and on the electron concentration in the crystal. It is concluded from these results that, in GaAs, diffusion of the Si donor species occupying Ga sites Si Ga+ is governed by the triply negatively charged Ga vacancies, VGa3-. The present VGa3--dominated SiGa+ outdiffusion diffusivity values are, however, larger than those obtained under Si indiffusion conditions by many orders of magnitude. A tentative explanation of this large difference is given in terms of an undersaturation of VGa3- in intrinsic GaAs during indiffusion experiments and of a supersaturation of V Ga3- developed during the outdiffusion of Si from n-type, Si-doped GaAs.