We have investigated the surface optical transitions on (NH4)(2)S-passivated, Na2S-passivated, and unpassivated GaAs as a function of annealing temperature. The change of the reflectance anisotropy (RA) spectrum allows us to distinguish several annealing stages, which we identify as desorption of the protective overlayer, desorption of residual impurities, and breaking of Ga-S bonds. For the case of impurity desorption, we find a very good connection between the increase of dimer concentration, as found from the RA spectrum, with the decrease of impurity concentration, as monitored by Auger spectroscopy. We find significant differences between the Na2S- and the (NH4)(2)S-passivated surfaces. Unlike the Na2S-passivated surface, most of the arsenic atoms situated on the (NH4)(2)S-passivated surface before annealing are found to be dimerized, so that they are not involved in any chemical bonds with the overlying protective layer. Thus, in this last case, As-S bends detected by photoemission spectroscopy are situated in the protective overlayer.