Data are presented demonstrating that the lateral wet oxidation of Al(Ga)As layer is strongly influenced by its thicknesses and heterointerface structures as well as Al compositions. The oxidation length decreases rapidly with decreasing AlAs thickness in the range of xGa1−xAs decreases quickly with decreasing Al composition, providing a high degree of oxidation selectivity. AlGaAs layers on both sides of AlAs layer reduce the lateral oxidation rate which is enhanced by the stress induced by oxidized AlAs.