Methyl tertiary butyl ether (MTBE) is a flammable, volatile, and colorless liquid fuel additive that is manufactured by the chemical reaction of methanol and isobutylene. It is one of a group of chemicals referred to as oxygenates, because they raise the level of oxygen in gasoline. Oxygen helps gasoline burn more completely, thus reducing emissions.

MTBE has a strong odor similar to a general anesthetic. Humans can detect it by smell at very low concentrations: 53 parts per billion (ppb) in air and 20 to 40 ppb in water. MTBE is very soluble in water and more soluble than other gasoline constituents. It is also persistent and resistant to degradation.

As a gasoline additive, it is used to increase the octane level (replacing the use of lead additives) and to reduce vehicular emissions of carbon monoxide (CO) and ozone-forming pollutants. It was first used in...