Direct measurements of the desorption rate of gallium from GaAs and (Al,Ga)As during growth by molecular beam epitaxy at high temperatures have been made by modulated beam mass spectrometry. The activation energy for desorption is dependent upon the nature of the site from which the gallium is lost. From free gallium atoms not incorporated into the lattice, behavior similar to that encountered under equilibrium conditions for gallium over gallium or gallium over GaAs is observed. For gallium lost from the GaAs lattice the apparent activation energy is higher and is influenced by the arsenic flux reaching the surface.