The relationship between the Al mole fraction (x) and the angular separation (&Dgr;&thgr;) of film and substrate maxima in double axis x‐ray diffraction rocking curves for strained epitaxial AlxGa1−xAs films on (110)‐ and (111)‐oriented GaAs substrates was determined. It was derived from measured &Dgr;&thgr;’s for 5000‐A˚‐thick strained epitaxial AlAs films grown by molecular beam epitaxy on (110)‐ and (111)‐oriented GaAs substrates and published values of the AlAs and GaAs lattice parameters and GaAs elastic constants. The critical thickness for relaxation appears to be significantly less for (111)‐ than for (100)‐ and (110)‐oriented films.