We have developed an atomistic model for the growth of GaAs and AlGaAs and have simulated the MBE growth of both the normal (AlGaAs on GaAs) and the inverted (GaAs on AlGaAs) structures along (100) direction using Monte Carlo techniques. We assume the growth to occur under anion overpressure with As2molecular species as the anion source and Ga and Al atoms as the cation sources. We find that some of the differences in the quality of the two interfaces can be explained on the basis of the surface kinetics operational for the two kinds of cations. In our model there is a considerable interlayer surface migration for the Ga atoms due to the relatively weak Ga–As bond compared to the Al–As bond. For comparable substrate temperatures the stronger Al–As bonds lower the interlayer diffusion for Al atoms. The role of this key kinetic step, namely, the interlayer surface migration on the quality of the growth front profiles of GaAs and AlGaAs as well as their interfaces is examined.