Blue Def 2.5 Gal. Diesel Exhaust Fluid is a mixture of 32.5% high purity synthetic urea and 67.5% deionized water that is used in selective catalytic reduction systems on diesel engines. Def is the primary ingredient used to help convert NOx to harmless nitrogen and water. Blue DEF is stable, colorless, non-flammable, non-toxic and is classified as minimum risk for transportation. Conforms to the ISO-22241-1 specification for DEF, is API registered and meets or exceeds OEM specifications.