The etching characteristics and physical/chemical mechanisms of Au thin films in Cl2/Ar plasma were investigated. It was found that an increase of the Ar content in Cl2/Ar plasma under constant pressure and input power conditions leads to an increasing etch rate of Au, which reaches a maximum value at 80%Ar/20%Cl2. X-ray photoelectron spectroscopy of the etched surfaces indicated the accumulation of reaction products in a chlorine-rich plasma. A proposed zero-dimensional model of volume kinetics, which involved the Langmuir probe data for electron temperature and electron density, showed monotonic change of both densities and fluxes of active species such as chlorine atoms and positive ions. In contrast, analyses of surface kinetics showed the possibility of nonmonotonic etch rate behavior due to the concurrence of physical and chemical factors in ion-assisted chemical reaction. © 2003 American Vacuum Society.