Near infrared Brillouin scattering was used to determine all three elastic constants, and hence Poisson’s ratio &ngr;=C12/(C11+C12) of epitaxial AlxGa1−xAs layers on GaAs. The AlxGa1−xAs layers were grown by metalorganic chemical vapor deposition and molecular beam epitaxy. We found evidence for a slight bowing in all elastic constants versus Al compositionx. The elastic constants of AlAs were determined to beC11=119.9 GPa±1.2 GPa,C12=57.5 GPa±1.3 GPa, andC44=56.6 GPa±0.7 GPa. From x‐ray measurements and the Poisson ratio &ngr;=0.324±0.004 the relaxed (cubic) AlAs lattice constant is found to beaAlAs=5.661 72 A˚±0.000 08 A˚. ©1995 American Institute of Physics.