Electron microscopy studies of annealed GaAs layers grown by molecular beam epitaxy at low temperature (200 °C) were used to monitor growth of As precipitates. Ostwald ripening kinetics was used to deduce a migration enthalpy of 1.4±0.3 eV for the diffusion mediating defect. A conclusive picture of the dominant diffusion mechanism can be given, attributing this value to the migration enthalpy of gallium vacancies (VGa), which is well established by other experiments. The present studies indicate that growth of As precipitates is driven by supersaturation ofVGa. ©1995 American Institute of Physics.