The limits of inflammability of hydrogen cyanide – air mixtures at room temperature and approximately 1 atm. pressure, have been determined to be 7.8 and 42.4 volume % of hydrogen cyanide. The effect of the addition of cyanogen chloride, phosgene, chloroform, hexane, heptane, and methyl chloroformate upon these limits has been investigated. It was found that the minimum molar ratio of added vapor to hydrogen cyanide that will produce a mixture that never becomes inflammable when progressively diluted with air is equal to 4.7 for cyanogen chloride, 0.86 for phosgene, and 1.1 for chloroform. All mixtures of hexane, heptane, and methyl chloroformate with hydrogen cyanide become inflammable when suitably diluted with air. For these systems, Le Chatelier's law, dealing with the composition of limit mixtures of two inflammable gases with air, was found to hold.