Heres a link www.eps.gov/mtbe/ that helps to explain.

Also, as a comment, here in the cesspool city of missouri, they use MTBE in the winter and I get 2-5 mpg LESS than with regular formulated gasoline. So I wonder... milage is less,therefore more gasoline is burned= more emissions. Even if a vehicle emits less pollutants with mtbe gas per volume burned, does the overall decrease in emissions balance out? I dont think anybody knows.