Ga self-diffusion in gallium phosphide (GaP) is measured directly in isotopically controlled GaP heterostructures. Secondary ion mass spectroscopy (SIMS) is used to monitor intermixing of69Gaand71Gabetween isotopically pure GaP epilayers which are grown by molecular beam epitaxy (MBE) on GaP substrates. The Ga self-diffusion coefficient in undoped GaP is determined to beD=2.0cm2 s−1exp(−4.5 eV/kBT)between 1000 and 1190 °C under phosphorus-rich condition. The self-diffusion entropy is found to be ∼4kB.©1997 American Institute of Physics.