The results of a study of the sensitizing action of chloropicrin upon the hydrogen-oxygen reaction suggested that chloropicrin might also sensitize the hydrogen-chlorine reactions by providing chlorine atoms which would initiate the reaction chains. Experimental work is described which shows that the addition of small proportions of chloropicrin lowers the normal ignition boundary of hydrogen-chlorine mixtures to the temperature range 100 to 200 degrees C. This sensitized boundary separates exceedingly rapid and complete reactions from slow reactions which are further retarded during their course by the formation of nitrosyl chloride from the chloropicrin. At higher temperatures (330 to 370 degrees C) the decomposition of the chloropicrin is sufficiently rapid for the inhibiting action of the nitrosyl chloride to become prominent during the entry of the gases, and the ignition boundary rises with increasing temperature. It rises more sharply with a greater proportion of chloropicrin in the mixtures entering the vessel, and when a longer portion of the entry tube is heated. The sensitized ignitions are suppressed by the addition of small quantities of nitrosyl chloride. A remarkable 'backfire' phenomenon, in which ignitions travel back to the cold mixing vessel but not into the hot reaction vessel, is described and shown to be in agreement with the assumption that reaction chains are started by chlorine atoms formed from chloropicrin and are abruptly terminated by nitrosyl chloride also formed from chloropicrin. Experimental work upon the effects of nitrosyl chloride upon the boundary of ignition of mixtures of hydrogen and chlorine show that below 400 degrees C it acts as an inhibitor, but above 400 degrees C it has a slight sensitizing effect.