Abstract: Provided is a novel semiconductor processing system. The system includes a process chamber for treating a semiconductor substrate with one or more process gases comprising water vapor, means for delivering the water vapor or one or more precursors thereof to the process chamber, an exhaust conduit connected to the process chamber, an absorption spectroscopy system for sensing water vapor in a sample region, and a control system which controls water vapor content in the process chamber. Also provided is a method for controlling the water vapor level in a semiconductor process chamber. The system and method allow for measurement and control of the water vapor level in a semiconductor processing chamber in which water vapor is present as a process gas.