Process and environmental gas analyses have been available for a number of years for regulatory, process-control, and quality-improvement purposes. In-line characterization of gases in real time involves a combination of speed, multicomponent analysis, reliability, and accuracy. This article describes an innovative analytical method to measure low concentrations of Cl-2, and HCl in water-vapor-saturated conditions. We discuss the essential components of the setup, analytical method, calibrations, and testing, demonstrating the advantages of this method over existing analytical techniques.