The concentration and distribution of chlorine incorporated into silicon substrates during reactive ion beam etching is investigated. Rutherford backscattering spectrometry and Auger sputter profiling are employed to analyze the single crystal silicon samples. Data are presented for samples etched at various energies, current densities, and angles of incidence. The effect of temperature, dopant concentration, and crystalline orientation are also discussed. The amount of chlorine retained by the substrate decreases sharply with increasing energy and temperature. The quantity of chlorine retained increases linearly with current density, and is insensitive to doping level and orientation. Chlorine incorporation increases with angle of incidence asA/cos (bθ), where θ is measured with respect to the surface normal. Samples etched at low energies show surface roughening and thick oxide growth. The data are in large part accounted for by a combination of direct and recoil implantation. However it is argued that radiation enhanced diffusion is responsible for results obtained at high current densities and large angles to the beam. Implications for reactive ion etching are discussed.