A real‐time reflectance‐difference spectroscopy (RDS) study of surface reactions of trimethylgallium (TMGa) and tertiarybutylarsine (TBAs) with (001)GaAs surfaces in an ultrahigh vacuum environment is reported. These studies reveal several phases of atomic layer epitaxy of GaAs using TMGa. A model consistent with various kinetics studies is established to explain the distinct behavior observed in RDS during TMGa exposures. It is shown that optimal growth conditions can be achieved through RDS monitoring. The self‐limiting mechanism which occurs in TMGa exposure cycle is believed to result from both selective adsorption and reaction of TMGa at As atoms and Ga vacancy induced Ga‐rich surface reconstruction. It is also shown that TBAs is a promising arsenic source for atomic layer epitaxy.