Abstract Reflectance anisotropy spectroscopy (RAS) oscillations during molecular beam epitaxy (MBE) growth of singular GaAs(001) were studied at substrate temperatures between 500°C and 610°C. RAS spectra were measured and related to surface reconstructions determined by reflection high-energy electron diffraction (RHEED). The RAS signal sensitively monitors surface changes when growth is initiated. With opening of the Ga shutter an immediate change in the RAS response indicates the modified As adsorption/desorption equilibrium due to the presence of Ga atoms on the surface. During growth of the first GaAs monolayers RAS oscillations mirror the oscillation of the monolayer step density in island growth mode. From all experimental evidence it must be concluded that the step density modulates the As dimer coverage because As desorbs predominantly in regions close to the island boundaries.