Reflectance anisotropy spectroscopy (RAS/RDS) so far has been mostly used for surface studies on Ill-V semiconductors. In the work reported here it was applied to the GaAs-AlAs (001) interface for measuring the interface dielectric anisotropy (IDA) by in-situ experiments performed in molecular beam epitaxy (MBE). From the measured RAS spectra of very thin AIAs overlayers (1-40 monolayers) on GaAs(001) the different contribution of surface, bulk and interface were separated. For the analysis, a three-layer model was used consisting of a single AlAs layer sandwiched between a well-separated and anisotropic interface layer and a surface layer. The upper limit for the 'thickness' of the surface and interface layers could be determined. Finally, an improved analytical formula for the deconvolution of surface and interface anisotropies is derived. (C) 2004 Elsevier B.V. All rights reserved.