P-Series fuel is a unique blend of natural gas liquids (pentanes plus), ethanol, and the biomass-derived co-solvent methyltetrahydrofuran (MeTHF). P-Series fuels are clear, colorless, 89-93 octane, liquid blends that are formulated to be used in flexible fuel vehicles (FFV's). P-Series are designed to be used alone or freely mixed with gasoline in any proportion inside the FFV's gas tank. These fuels are not currently being produced in large quantities and are not widely used.

Since 1992, when the Energy Policy Act (EPAct) was passed, only one new fuel has been recognized as an alternative fuel under the EPAct petitions provision. P-Series fuels were added to the list of alternative fuels in 1999.