A reaction model for the epitaxial growth of GaAs by chemical beam epitaxy using triethylgallium and tris(dimethylamino)arsenic is presented. The model is developed by properly combining surface decomposition mechanisms of the two metalorganic precursors on GaAs(100) surfaces. Arguments are outlined regarding a technique for obtaining the absolute group-V incident flux directly from experiment. For the purpose of routine applications of the model, inputs are all readily obtainable From experimental setup. Computer simulations based on the model are carried out to make comparisons with the experimental observations for this growth system. The model is shown to provide very good agreement with the growth kinetics observed. Sensitivity analyses are performed to develop an understanding of the growth kinetics. Transient growth of this system is simulated to look into the time scale for growth to reach a steady state.