Exxon Mobil agreed to install equipment on refineries in five states to cut emissions of sulfur dioxide and nitrogen oxides by 75 percent in coming years. Those pollutants are linked with acid rain, smog and asthma.

"We believe that the settlement is in the best interest of the company and support the continued trend of emission reductions that has occurred at our refineries," an Exxon Mobil spokeswoman said.

The settlement is for alleged violations of the "New Source Review" section of the Clean Air Act, which requires refineries and utilities to install pollution-reduction equipment when they upgrade and expand their facilities.

In the settlement, Exxon Mobil denies that it violated any laws or regulations.

The Bush administration has proposed weakening clean air rules to allow US industry to expand and retool without such investments. Exxon Mobil said it supports the proposed rule change, which has been frozen by a court order.

Exxon's refineries affected by the settlement represent about 11 percent of US refining capacity, the government said.

They include Exxon Mobil's giant refineries in Baytown, Texas, and in Baton Rouge, Louisiana -- the nation's two biggest. Other refineries in Texas, Louisiana, Montana, California and Illinois are also affected.

"The emissions reductions required by this settlement will lead to cleaner air and significant environmental and public health benefits," said acting US Assistant Attorney General Kelly Johnson.

In recent years the government has reached settlements with 16 other US refiners, which include 77 percent of US refining capacity.