Diesel exhaust fluid, carried properly. If you run a modern diesel and a DEF light comes on 200 miles from the nearest truck stop, that's a limp-mode problem — this is the fix.Specifications Capacity 2 Gallon Contents DEF (diesel exhaust fluid) Material Roto-molded HDPE, compounded specifically for DEF Warranty 10-year leak-free guarantee Included Pour spout Made in USA Do not decant DEF into a fuel packThe plastic compound in each RotoPax container is specific to its labelled contents. DEF is corrosive to materials that handle gasoline perfectly well, and gasoline degrades the DEF. Using the wrong container compromises the product and voids the warranty. Buy the right pack.MountingShares the standard RotoPax mounting pattern, so it stacks and mounts alongside your fuel and water packs. Beyond RotoPax's own Standard, DLX and LOX Pack Mounts, dozens of manufacturers build brackets to it — including our Heavy-Duty RotoPax Spare Tire Mount, plus Front Runner, Eezi-Awn, Leitner, Axia Alloys, Xtrusion, TrailRax and RockSlide.VentingVent at least once a day, and again for every 10°F of temperature change or every 1,000 ft of elevation gain. Store the container completely full or completely empty.