Si thermal etching studies have been performed using pure Cl-2 in an ultrahigh vacuum rapid thermal chemical vapor deposition reactor in the temperature range of 650-850 degrees C and the flow rate range of 1-10 seem which corresponds to a pressure range of 0.5-3.5 mTorr. The effects of temperature and Cl-2 flow were investigated with thermodynamic equilibrium calculations performed to determine possible reaction pathways. The effect of adding H-2, up to 500 sccm, on Si etch rates at 800 and 850 degrees C was also obtained experimentally. Thermodynamic equilibrium calculations were used to support the experimental results and determine the reaction by-products. It is proposed that SiCl2 equilibrium partial pressure can be used as a means to compare the etching ability, thus the selectivity, of different selective Si processes. The results from the etching studies were used to explain the behavior of Si epitaxy growth rate from the Si2K6, H-2, and Cl-2 system in the 650-850 degrees C, 22-24 mTorr processing regime. The implications of the etching studies for selective silicon epitaxy with the Si2H6 and Cl-2 chemistry are discussed and then extended to the SiH2Cl2 based chemistry.