We report on <i>in situ</i> Reflectance Difference Spectroscopy measurements carried out on GaAs (001). Measurements were
performed at temperatures of 580 °C and 430 °C, in both n and p-type doped films and for both (2x4) and c(4x4)
reconstructions. Samples employed were grown by Molecular Beam Epitaxy with doping levels in the range from
10<sup>16</sup> - 10<sup>19</sup> cm<sup>-3</sup>. We demonstrate the potential of Reflectance Difference Spectroscopy for impurity level determinations under growth conditions.