[en] We report on a reflectance anisotropy (RA) spectrometer capable of measuring reflectance spectra on the 100 ms time-scale and sensitivity in the upper 10⁻⁴ range. A multichannel lock-in amplifier was used to acquire 32 wavelengths RA spectra covering the 2.25–3.85 eV photon energy range, where the E ₁ and $E_1+{\mathrm{\Delta <!--\; ??\; -->}}_1$ transitions of GaAs and other technologically relevant III–V semiconductor are located. The RA spectra recorded during the first stages of the GaAs homoepitaxial deposition are presented for the first 0.38 monolayers of growth, showing significative changes in the lineshape with low noise. Thanks to the capabilities of this instrument, it is possible to observe in detail, in terms of the evolution of RA spectra, the processes carried out during the migration of surface reconstruction between two stable phases present in the homoepitaxial growth of GaAs. (paper)