Two fundamental requirements for RIE are the formation of nearly volatile etch products and sufficiently high physical bombardment to remove all substances on the surface. In this study, the GaAs wafer was in-situ pretreated with NH3 or CHF3 plasma prior to actual etching process. The main etchants are CCl2F2 and SiCl4. By adding these additives to the main etch gases, the resulting etch performance was significantly affected. For instance, DC self-bias of CCl2F2 plasma is relatively low and can increase with such gas addition, thus the etching properties related to physical bombardment change too. CHF3 improve GaAs etch rate in CCl2F2through increasing concentration of reactive chlorine-containing species. While CHF3 enhance etch rate in SiCl4 plasma. The as etched samples were examined with X-ray photoelectron spectroscopy. Details of the experimental results will be described.