Peak 55-Gallon Blue Def Diesel Exhaust Fluid. It is a stable, colorless, non-flammable, non-toxic mixture of 32.5% high purity synthetic urea and 67.5% deionized water. Used in selective catalytic reduction systems on diesel engines, Def helps convert NOx to harmless nitrogen and water. It is classified as minimum risk for transportation and conforms to the ISO-22241-1 specification for Def. API registered and meets or exceeds OEM specifications.