This method can be used for the determination of chlorine in compressed hydrogen gas or for the determination of hydrogen chloride in other gas systems, such as refinery recycle gases. The method as applied to compressed hydrogen does not distinguish between chlorine and hydrogen chloride and the results are expressed in terms of mol ppm of chlorine. For recycle gas analysis, the chlorine is in the form of hydrogen chloride and is expressed as such. Two to six cubic feet of sample is sufficient to detect a chlorine or hydrogen chloride content equal to or greater than 1 mol ppm. Chlorine content in the range of 0.1 to 1.0 mol ppm can be determined by the use of a proportionately larger sample.