Chlorine atoms were used to activate a Si(100) surface forming a self-limited monolayer of primarily silicon oxide as shown by X-ray photoelectron spectroscopy. A saturation coverage of 0.75 monolayers of chlorine atoms was deposited on Si(100) by exposing a surface at 300 K to Cl2 gas at 10 Torr under ultraviolet illumination. The oxide layer was formed by exposing the chlorine terminated surface to water vapor at 200 Torr and temperatures ranging from 325 to 373 K, followed by annealing to 800-900 K. The coverage of oxygen resulting from 325 K H2O exposures was directly correlated with a decrease in the Cl coverage and ranged from 0.2-1 monolayer. Complete removal of surface chlorine was achieved by higher temperature water exposures. The deposition of Ti using TiCl4 showed that -0.5 silanol (SiOH) group/nm2 were present on the surface. The activation of a Si(100) surface using a halogen demonstrates an alternative approach to grow ultrathin interfacial layers with monolayer control at low temperature.