Atomic layer epitaxy (ALE) of GaAs using trimethylgallium and triethylgallium by chemical beam epitaxy is reported. Reflection high‐energy electron diffraction is employed to monitor the formation of each individual monolayer directly. A new (4×8) surface reconstruction, characteristic of an ordered chemisorbed molecular Ga monolayer, is observed for the first time. Each Ga atom in this transitional layer has at least one alkyl radical cleaved away. The stability of this transitional stage varies depending on the alkyl group involved and the temperature. Dynamical evolution of this adsorbed layer is described. The chemical properties of this overlayer dictate the GaAs ALE growth conditions.