The influence of hydrogen adsorption on the surface order, dielectric function, and dielectric anisotropy was investigated for the three main reconstructions of the GaAs(100) surface [c(4X4), (2X4), (4X2)] by low-energy electron diffraction, spectroscopic ellipsometry, and reflectance difference spectroscopy. For all reconstructions, low hydrogen exposure removes the surface dimers, whereas surface roughening is observed at high hydrogen exposures. However, detailed differences can be resolved depending on the surface reconstruction. On the As-rich c(4X4) surface, the outermost As-dimers are removed at low H-exposures leaving behind a still As-rich surface of (1X1) symmetry. Thereafter, more As is removed and a new surface anisotropy develops due to the exposed Ga-dimers arranged in a mixture of (1X2) and (root 2X root 2)-reconstructions. For H-adsorption on the (2X4)- and (4X2)-surfaces, in contrast, only the subsequent reduction of the surface reconstruction is observed with increasing exposure, but no structural rearrangement. The removal of the As- [Ga-] dimers modifies the surface symmetry from (2X4) [(4X2)] through (1X4) [(4X1)] to (1X1). Finally, for large exposures, inhomogeneous surface etching by hydrogenation leads to rough, disordered surfaces for all three reconstructions.