The dominant factors governing reactive ion etching of Si, SiO, and SiO2 thin films during bombardment with energetic Ar ions in a Cl atmosphere were investigated. Etch rates were determined in-situ by measuring weight loss as a function of ion fluence for films deposited on a quartz micro-balance. Measurements were made as a function of ion energy and Cl pressure using a Kaufman gun for ion energies from 300 to 1500 eV and an electron cyclotron resonance microwave ion source for ion energies from 50 to 600 eV and for Cl pressures ranging from 0.006 to 0.05 Pa. Results were compared to sputtering in Ar alone. The results indicated that the presence of the C1 enhanced the etch rate for all three materials, with the degree of enhancement being inversely proportional to the oxygen content of the film. It was also discovered that there was an optimum pressure for enhancement of the etching and that at the highest pressures, the etching could actually be suppressed. These results are discussed in terms of physisorbed gas layers, the thickness of which depends on the balance between the flux of energetic particles and the gas flux.