We have studied the oxidation in water vapor of thick (0.5–2 &mgr;m) AlGaAs layers with aluminum contents ranging from 48% to 78% grown by molecular beam epitaxy. The oxidation rate was measured as a function of the aluminum content, the temperature and the flow rate of the N2carrier gas supplying the water vapor. The oxide films proved to be mechanically very stable and the growth was found to be linear with time also in the case of an excessive supply of water vapor. The activation energies for the wet oxidation of AlGaAs were determined to increase from 1.1 to 1.8 eV for the Al contents decreasing from 78% to 48%. ©1995 American Institute of Physics.