Abstract Experimental results on the application of reflectance anisotropy spectroscopy (RAS) to the monitoring of (reactive) ion etching of monocrystalline semiconductor samples are described. To show the potential of this technique RAS signals collected during etching of GaAs/AlxGa1−xAs multilayer samples are compared to RAS data obtained before during molecular-beam epitaxial (MBE) growth of these very samples. A change of the RIE-RAS spectrum can be attributed to a change of material composition. And the current etch depth can be monitored with an accuracy at least down to several tens of nanometers – f. e. by recording the average reflected intensity.