Mustard gas is a vesicant or blistering agent. Its code name is HD. This agent was heavily employed in World War I and was stockpiled in World War II.

A large family of related mustard gases have been prepared, of which Bis (2-chloro-ethyl) sulfide, is the basic example. This is a relatively nonvolatile, water insoluble, oily liquid with a faintly agreeable odor. It has durability under normal climatic conditions for several days.