The formation of the Ge/GaAs(001) interface has been investigated following the transformation of an As-dimer terminated GaAs(001)(2 X 4) surface into a Ge-Ga-dimer terminated (1 X 2) reconstruction and the subsequent deposition up to 10 ML of Ge. The modification of the surface atomic geometry and the related electronic structure has been monitored by reflectance anisotropy spectroscopy (RAS) and low-energy electron diffraction. Experimental results are compared to density-functional-theory-local-density-approximation and tight-binding calculations of the surface structure and optical response, respectively. The comparison between calculated and measured RAS spectra allows us to show that the (2 X 4) structure transforms into a well-ordered (1 X 2) passing through a disordered (2 X 4) phase while a previously proposed intermediate (2 X 1) structure is ruled out. At higher Ge coverages, surface and Ge/GaAs-interface contributions to the optical spectra are separated by surface modification through exposure to atmosphere. A interface contribution is identified between 1.5 eV and 2.5 eV, almost identical in line shape and amplitude to the RAS features on the Ge-Ga-dimer terminated GaAs surface. This finding demonstrates that the backbonds of the Ge-Ga-dimers, present at the Ge-Ga-dimer terminated surface as well as at the Ge/GaAs interface, determine the optical anisotropy, whereas the Ge-Ga-dimer bond itself does not contribute significantly. [S0163-1829(99)12315-4].