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 of Ga69 and Ga71 between 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 be D=2.0 cm2 s−1 exp(−4.5 eV/kBT) between 1000 and 1190 °C under phosphorus-rich condition. The self-diffusion entropy is found to be ∼4 kB.