Device quality GaAs was grown in a conventional Organometallic Chemical Vapor Deposition (OMCVD) reactor, using sequential group III and V reactant gas exposures typical of Atomic Layer Epitaxy (ALE). The importance of gas phase concentration transients during the ALE cycles was revealed by systematic investigations of the effect of the sequences used, for the cycles, on impurity incorporation as well as on the growth rates. In this study, we attempt to quantify the effects of such transients by solving the diffusion equation for the reactant gases, with initial conditions specific to ALE. We used this model to calculate the time dependence of the reactant gas concentration at the growing surface. This quantitative study gives us new insights into the ALE technique and confirms that the V/II ratio at the substrate surface can be controlled by the choice of the gas sequence.