Undoped69GaAs/71GaAs isotope superlattice structures grown by molecular beam epitaxy onn‐type GaAs substrates, doped by Si to ∼3×1018cm−3, have been used to study Ga self‐diffusion in GaAs by disordering reactions. In the temperature range of 850–960 °C, the secondary ion mass spectrometry (SIMS) measured Ga self‐diffusivity values showed an activation enthalpy of 4 eV, and are larger than previously compiled Ga self‐diffusivity and Al‐Ga interdiffusivity values obtained under thermal equilibrium and intrinsic conditions, which are characterized by a 6 eV activation enthalpy. Characterizations by SIMS, capacitance‐voltage (C‐V), and transmission electron microscopy showed that the as‐grown superlattice layers were intrinsic which turned intoptype with hole concentrations of ∼2×1017cm−3after annealing, because the layers contain carbon. Dislocations of a density of ∼106–107cm−2were also present. However, the factor responsible for the presently observed larger Ga self‐diffusivity values appears to be Si out‐diffusion from the substrate, which was determined usingC‐Vmeasurements. Out‐diffusion of Si decreases the electron concentration in the substrate which causes the release of Ga vacancies into the superlattice layers where they become supersaturated. This Ga vacancy supersaturation leads to enhanced Ga self‐diffusion in the superlattice layers.