We report on in situ RD measurements of In0.3Ga0.7As/GaAs epitaxial layers, 0-75 Angstrom thick, grown by Molecular Beam Epitaxy (MBE) on GaAs (0 0 1). A plot of the amplitude of the RD spectrum at 2.5 eV versus epilayer thickness shows that it rises sharply when a film thickness of about 38 Angstrom is reached. Comparison with RHEED measurements shows that such rise correlates well to the onset of 3D growth. RD line shape calculations show that the experimental RD line shapes can be described on the basis of InGaAs islands under an anisotropic strain. Results presented here show the high potential of RD spectroscopy as an in situ probe to the study of In0.3Ga0.7As/GaAs growth processes. (C) 2003 Elsevier B.V. All rights reserved.