We report the in situ optical monitoring of AlAs/GaAs structure elaborated and etched by CCl4 in metalorganic vapour phase epitaxy reactor. We describe the. modelling of experimental reflectivity response to determine the properties of AlAs/GaAs structure such as the refractive index and growth/etch rate. Etching rates of AlAs and GaAs increase with the increase of the flow of CCl4 or the substrate temperature. In most of the cases, the etching rate of GaAs is higher than that of AlAs. This fact might be related to the difference between the chloride species responsible to the mechanism of the etching. The thermodynamic analysis of the two mixtures (solid GaAs + H-2 + CCl4 and solid AlAs + H-2 + CCl4) suggests that GaCl and AlClx (x = 1 - 2) are the species responsible for the etching of GaAs and AlAs by CCl4, respectively. The surface flatness of GaAs and AlAs etched by CCl4 is also qualitatively monitored in situ by laser reflectometry. Specifically, we have studied the evolution surface morphology with etching rate. The smoothest surface is reached by optimizing the etching parameters. (C) 2003 Elsevier B.V. All rights reserved.