Reflectance anisotropy (RA) spectrosocpy has been used to study at 300 K the intrinsic optical transitions on sulfur-passivated surfaces. The spectra allow to follow the modification of surface dimers after annealing at a temperature Ta. For Ta≊440 °C, the S-treated surface is covered by arsenic and sulfur dimers oriented along the [11̄0] direction. Upon subsequent heating, sulfur is desorbed and there appear gallium dimers oriented along [110]. Sulfur passivation has allowed to obtain stable surface structures, with a strongly reduced band bending, which are consistent with the known (2×1) and (4×6) reconstructions.