Also known as methanethiol, mercaptan produces the pungent smell we associate with natural gas. Because natural gas is odorless and colorless, it is necessary to inject mercaptan so that leaks can be detected. Mercaptan is flammable and can be detected by humans in extremely small quantities. In the gas business, mercaptan is sometimes referred to as odorant.

In most instances mercaptan is injected at the citygate since this is typically where the natural gas system approaches homes and businesses. While some states require mercaptan to be injected into the transmission system as gas enters the state, this is not typical as leaks or other problems on a transmission line are likely to be diagnosed from the pipeline’s SCADA system and not from someone smelling mercaptan.