The growth of indium-layers in the monolayer regime on As-rich (2X4)/c(2X8)- and Ga-rich (4X2)/c(8X2)-GaAs(100) surfaces has been investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and reflectance anisotropy spectroscopy (RAS). Clean (2X4)- and (4X2)-reconstructed surfaces were prepared in UHV by thermal desorption of a protective arsenic layer deposited on top of GaAs(100) surfaces grown in a molecular beam epitaxy (MBE)-system. For the (2X4) reconstruction, the STM images show As-dimer rows consisting mostly of two dimers per unit cell in the outermost layer. Besides a large number of kinks in the dimer rows, a surface roughness corresponding to about three bilayer steps (8.4 Angstrom) is observed due to multiple layer nucleation during MBE growth. After deposition of indium and subsequent annealing to approximately 450 degrees C a well ordered reconstruction with c(8X2)-symmetry is established. The STM images show a pattern consisting of kink free, alternating straight and broken rows oriented along the [110]-direction attributed to In-dimer rows. Remarkably, the surface roughness of these In-terminated surfaces is strongly reduced. For the Ga-terminated (4X2) surface reconstruction, STM results show a surface terminated with Ga-dimer rows, free of kinks, which appears flat over large areas similar to the In-terminated surface. After deposition of indium and annealing to 450 degrees C, LEED, RAS and STM reveal a surface structure very similar to that formed on the (2X4)-surface. Thus we conclude that in both eases a structure with In-dimer-rows on top of an As-layer (second layer) is formed. The smoothing of the surface roughness is a consequence of the larger mobility of the group III surface atoms, following the As desorption.