Up to now most of the experimental work regarding the adsorption of organic molecules has been concerned with silicon. Here we study the interface formation on a III-V-semiconductor, GaAs(001). We show that reflectance anisotropy spectroscopy (RAS) is a sensitive technique for investigating the interface formation between organic molecules and semiconductor surfaces. With RAS it is possible to determine the surface reconstruction and the structural changes at the interface during the deposition of organic molecules. These changes and the underlying adsorption process are discussed here for the adsorption of cyclopentene on GaAs(001)c(4x4), (2x4) and (4x2).