From What Is Mustard Gas Made?
Mustard gas, also known as sulfur mustard, is the organic compound (CI-CH2-CH2)2S that is created when sulfur mustard is synthesized by treating sulfur dichloride with ethylene. There is other methods for creating this toxic chemical agent, but they all typically attack the same areas of the human anatomy, such as the eyes, skin, lungs and gastro-intestinal tract.
In its pure form, mustard gas is an odorless and colorless liquid which is said to be quite stable when stored properly. The term "mustard gas" was actually coined because one of the earliest variants actually produced a mustard like smell. The gas its self, when used as a weapon during wartime, has proved extremely effective in its ability to seep through clothing, bind to skin and work its way into the gastro-intestinal system and blood stream. Mustard gas causes terrible burns and blistering effects on the outside of the victim's body and also within as well.