The lawsuit claims that the oil companies have added increasing amounts of the additive to gasoline, even though they knew years ago it would contaminate water supplies.
"New Hampshire's groundwater and surface waters are under attack," Attorney General Peter Heed said at a news conference with Benson.
New Hampshire is the first state to sue gasoline companies over MTBE, Benson said, although municipal utilities have sued.
In California, Sacramento County and 10 water utilities filed suit last week against major gas companies over potential MTBE contamination. Utilities in the South Tahoe, California, area reached a $28 million settlement with oil companies last year over MTBE pollution.
Spokesmen for several oil companies, including Amerada Hess Corp. and ChevronTexaco Corp., said Monday they weren't familiar with the New Hampshire lawsuit and needed time to respond.
Heed called MTBE "the Houdini of pollutants" because the chemical is water-soluble and seems to be able to escape from ground tanks and pipes.
MTBE was added to gasoline to cut air pollution. In 1991, the state agreed with federal regulators to require gasoline suppliers to use additives to reduce air pollution in four counties as an alternative to tailpipe emission testing, the lawsuit said.
The oil companies chose MTBE instead of other additives, such as ethanol, that would have caused fewer problems, the suit said.
Senior Assistant Attorney General Maureen Smith said the state has spent millions investigating and cleaning up MTBE pollution and the costs are rising. She would not put a specific dollar amount on the costs or of the damages that the state is seeking.
The state is asking the court to hold the companies responsible for all costs, including investigative and cleanup costs.
The state environmental department said MTBE was detected in 15 percent of public water supplies in 2002.
Heed said preliminary figures for 2003 show that more than 40 percent of public water supplies in Rockingham County were contaminated.
"The problem appears to be worsening," Heed said.
In Washington, House and Senate negotiators trying to work out a federal energy bill have been considering whether to ban MTBE.