MTBE

MTBE (methyl tertiary-butyl ether) is a chemical compound that is manufactured by the chemical reaction of methanol and isobutylene. MTBE is produced in very large quantities (more than 200,000 barrels per day in the United States in 1999) and is almost exclusively used as a fuel additive in motor gasoline. It is one of a group of chemicals commonly known as oxygenates because they raise the oxygen content of gasoline. At room temperature, MTBE is a volatile, flammable and colorless liquid that dissolves rather easily in water.

MTBE has been used in U.S. gasoline at low levels since 1979 to replace tetra-ethyl lead as an octane enhancer to help prevent the engine from "knocking." Since 1992, MTBE has been used at higher concentrations in some gasoline to fulfill the oxygenate requirements set by Congress in Clean Air Act amendments.

Source

EPA MTBE FAQ webpage