Reflectance anisotropy spectroscopy was used to examine the surfaces of AlxGa1−xAs layers grown on GaAs(001) by molecular beam epitaxy, where the Al mole fraction was varied across the whole composition rangex=(0.0,0.25,0.50,0.75,1.0). All surfaces were also independently characterized using reflection high‐energy electron diffraction, and were found to exhibit ac(4×4) reconstruction. After initial changes in the spectra were observed on depositing very thin layers (≤20 monolayers), in the intermediate thickness range a regime was entered in which strong optical interference effects appeared. These effects are accurately accounted for using a four‐media model. For thicker layers (≥8000 monolayers), interference effects were seen to diminish and spectra representative of the surfaces of bulk AlxGa1−xAs were obtained. ©1995 American Institute of Physics.