The thermal desorption kinetics of Ga from initially As‐terminated Si(100) is investigated using laser‐induced fluorescence spectroscopy. During the Ga desorption process the surface does not remain fully As terminated because a significant fraction of the initial As coverage simultaneously desorbs with the Ga. For Ga coverages ≥ 0.55±0.1 ML (1 ML=6.8×1014atoms cm−2), Ga desorbs from As‐terminated Si initially by a fractional order kinetic mechanism. Lower Ga coverages desorb more slowly and by a first order mechanism. The results suggest that at high coverages Ga desorbs initially from islands, and then at lower coverages from a two‐dimensional (2D) layer. Based on the distinct differences between Ga desorption from bare Si(100) and the desorption of Ga from the 2D Ga/As interfacial layer, this layer is probably GaAs‐terminated Si(100). These results on the GaAs system are compared with those previously reported on the InAs system, which show that In interacts with As on Si(100) less strongly than does Ga.