Technically, mixing ammonia and bleach will not produce mustard gas. It is a common misconception that chlorine gas is mustard gas. While the two are similar, it takes more than just mixing the two household cleaners to make mustard gas. They must be mixed in the right parts and exposed to other chemicals to lower the pH and synthesize the correct reaction to get C4H8Cl2S, aka mustard gas. That said, of course, still DO NOT mix the two together! It will form chloramine, a very toxic chlorine gas.