An argon ion beam is combined with a Cl2gas jet to etch silicon. Auger electron spectroscopy is used to measure the surface coverage of chlorine simultaneous with the etch process. This steady state surface chlorine coverage is enhanced by an increase in the Cl2flux or a decrease in Ar ion flux. The coverage varies from 10% to 100% of saturation as the Ar+to Cl2flux ratio varies from 0.4 to 0.004. The Si etch rate increases with increasing Cl2flux giving an enhancement of as much as a factor of 3 over pure Ar+sputtering at 500 eV. A general model is presented which is based on the mass balance of ionic and neutral species to and away from the surface region. Assumptions are made to apply the model to the Ar++Cl2+Si system. Expressions for both the surface Cl coverage and silicon etch rate are then obtained as functions of the incident Ar+and Cl2fluxes. The model is fit to the data and accounts for all of the observed trends in surface coverage and etch rate.