On February 10, 1994, the EPA proposed the Information Collection Rule (ICR), which will require public water supplies to begin monitoring for several disinfection byproducts. Of particular interest is cyanogen chloride, a byproduct formed in chlorinated tap water due to the reaction between chloramines and naturally occurring organic substances (i.e. humic and fulvic acids). Until recently, standards for cyanogen chloride were not conveniently available to commercial laboratories. A lab in the past would have to handle this very toxic compound in a gaseous form using very special handling precautions. When cyanogen chloride is analyzed by gas chromatography/ion trap mass spectrometry under the conditions specified in EPA Method 524.2, it was found to elute from the column very close to an EPA regulated compound, vinyl chloride. Depending upon the individual purge and trap setup and GC column used, the retention time difference between the analytes ranges from 3 to 10 seconds. In the ion trap, the cyanogen chloride mass spectrum is virtually identical to that of vinyl chloride, which may easily lead an experienced GC/MS chemist to misidentify either compound. The purpose of this paper is to examine the mechanism and conditions that lead to the formation of a mass spectrum for cyanogen chloride that looks identical to that of vinyl chloride. Also, the design and water treatment chemicals used at a water treatment plant in Valparaiso, Indiana, will be discussed in relation to the formation of cyanogen chloride in treated water. Preliminary evidence indicates that the formation of cyanogen chloride in treated water may be due in part to reactions among the water treatment chemicals themselves.