The success In attaining atomic layer epitaxy (ALE) of GaAs depends critically on the choice of the Ga precursor. Three systems were examined: trimethylgallium (TMGa) and diethylgallium chloride (DEGaCI) both of which give ALE, and triethylgallium (TEGa) which does not. We compared the surface reactions of these compounds on GaAs(100) and concluded that there was no evidence for reaction selectivity between Ga and As sites to cause ALE. Site blocking by the ligands on the Ga precursors alone also could not provide a self-limiting Ga deposition for ALE. We found evidence of a new mechanism by which self-limiting deposition of Ga resulted when the incoming Ga flux by the adsorption of Ga precursors was counter-balanced by an outgoing flux of Ga containing reaction product. For TMGa and DEGaCI with which ALE is successful, the products are CH3Ga and GaCl, respectively. For TEGa, the corresponding compound C2H5Ga was not formed.