A new two-phase model for gallium desorption can quantitatively explain the change of the desorption energy as a function of the III/V ratio. In this model, the gallium loss rate is described as a phenomenon of simultaneous desorption from a Ga fluid state and a GaAs solid state. This behavior was experimentally verified by accurate thickness measurements of epitaxial Al xGa1-xAs layers by transmission electron microscopy. The results show the Ga loss rate is directly dependent only on the Ga coverage on the surface, while the desorption energy is independent of the aluminum concentration. By varying the III/V ratio, we found that the Ga desorption energy increases from 3.1 eV at low arsenic flux to 4.7 eV at high arsenic flux.