The etch‐rate ratio of oxides to nonoxides in inert gas ion milling systems can be increased by injecting a flux of halocarbon gas molecules onto the surface along with the inert‐gas ion beam. In order for the halocarbon to be effective, it must adsorb on the surfaces of interest and the halogon must form a volatile reaction product with the surfaces. The system Ar+/CCl4/Si and SiO2is used to illustrate this method.