A chemical processing system includes a processing chamber containing a chemical processing region and a gas injection system. The gas injection system includes at least one first gas injection orifice and at least one second gas injection orifice in communication with the chemical processing region to expose a substrate to mixed first and second process gases. Other embodiments of the chemical processing system can include a sensor to sense a mixing rate of the process gases or a shroud defining a portion of the at least one first gas injection orifice to control mixing of the process gases. A method of mixing process gas in a chemical processing region of a chemical processing system is provided in which a first process gas and a second process gas are injected into the chemical processing region and mixed. A mixture rate is sensed and used to control the mixing.