Mustard gas or sulfur mustard is a chemical compound belonging to a family of cytotoxic and blister agents known as mustard agents. The name mustard gas is technically incorrect: the substance, when dispersed, is often not actually a gas, but is instead in the form of a fine mist of liquid droplets. Mustard gases form blisters on exposed skin and in the lungs, often resulting in prolonged illness ending in death.

The active ingredient in typical mustard gas is the organosulfur compound bis(2-chloroethyl) sulfide. In the wider sense, compounds with the structural element SCH2CH2X and NCH2CH2X are known as sulfur mustards and nitrogen mustards, respectively. Such compounds are potent alkylating agents, which can interfere with several biological processes.

s(cream)