The MTBE controversy concerns Methyl tert-butyl ether (MTBE), a gasoline additive that replaced tetraethyllead.  MTBE is an oxygenate and raises gasoline's octane number. Its use declined in the United States in response to environmental and health concerns. It has polluted groundwater due to MTBE-containing gasoline being spilled or leaked at gas stations. MTBE spreads more easily underground than other gasoline components due to its higher solubility in water. Cost estimates for removing MTBE from groundwater and contaminated soil range from $1 billion to $30 billion,  including removing the compound from aquifers and municipal water supplies, and replacing leaky underground oil tanks. Who will pay for remediation is controversial. In one case, the cost to oil companies to clean up the MTBE in wells belonging to the city of Santa Monica, California is estimated to exceed $200 million.
Some U.S. states banned MTBE in gasoline. California and New York, which together accounted for 40% of U.S. MTBE consumption, banned usage of the chemical in gasoline, effective 2002 and 2004, respectively. As of 2007, 25 states had issued complete or partial bans on the use of MTBE.
The Energy Policy Act of 2005 prompted gasoline refiners to replace MTBE with ethanol.