The sputtering yield of Si when bombarded with a flux φAr+of low‐energy Ar+ions may be enhanced a few times when the Si surface is exposed simultaneously to Cl2fluxes φCl2about one order of magnitude larger than φAr+. The mechanism of this synergistic etching has been studied, using mass spectroscopy and time‐of‐flight techniques, for a φAr+of about 5×1014Ar+ cm−2 s−1at Ar+ion energies (Ep) from 0.25 to 5 keV, Cl2fluxes between 1015and 5×1016Cl2 cm−2 s−1andTin the range 300–625 K. As has been shown previously [J. Vac. Sci. Technol. A2, 487 (1984)] the main products of the synergistic reaction are atomic Si and Cl and molecular SiCl and SiCl2; the kinetic energy distributions of the molecular products consist of two parts, a Maxwell–Boltzmann and a collision cascade‐like distribution. The Maxwell–Boltzmann part decreases relative to the collision cascade‐like part whenTand/orEpare increased and/or φCl2is decreased. At the same time the collision cascade‐like part shifts to higher kinetic energies. These observations and the dependence of the relative contributions of the various products on φCl2,T, andEpcan be understood from the Ar+ion‐induced amorphization and Ar+ion‐induced mixing of adsorbed Cl into the top atomic layers of the Si, in which the molecular products SiCl and SiCl2are formed and also trapped, without further chemical binding to the surrounding bulk network; and for higherTalso by the competition between the thermal reaction, yielding SiCl4(and SiCl2), and synergistic etching. The yields of the molecular products SiCl and SiCl2show an increase relative to atomic Si and their binding energies are reduced when φCl2is enhanced and/orEpand/orTare decreased. It is suggested that this is due to an increasing concentration of Cl ion mixed into the top atomic layers of the Si.The experiments also suggest that the binding energy of atomic Si decreases systematically with increasing Cl concentration in the Si. The results show that the etching of Si with Cl2under simultaneous Ar+ion bombardment is not due to stimulation of the thermal reaction but to chemically enhanced physical sputtering.