The properties of organic molecular layers grown from perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on GaAs(001) substrates were investigated using reflectance anisotropy spectroscopy/reflectance difference spectroscopy (RAS/RDS). In an attempt to grow ordered organic layers GaAs(001) surfaces were modified with sulfur prior to the evaporation of PTCDA under ultrahigh vacuum conditions. The chalcogen modification results in a gallium sulfide-like surface layer terminated by S dimers, which shows a (2x1) low-energy electron diffraction pattern. The lines shapes of the RAS spectra of S modified surfaces show sharp derivative-like features at the E-1 gap and broad spectral features at higher energies likely related to E'(0) and E-2 gaps of bulk GaAs. For low PTCDA coverages the shape of the spectra in the energy range of the GaAs bulk features is unchanged which indicates a low interaction between substrate and organic layer. Additional features appear in the spectra for PTCDA coverages even below 0.3 nm which can be attributed to transitions between the highest occupied molecular orbital and the lowest unoccupied molecular orbital at 2.23 eV. While the sharp feature due to the E-1 gap of GaAs is essentially unaffected, the optical anisotropy at higher energies is increasing strongly with increasing PTCDA layer thickness which is due to interference effects as shown by a model calculation.