BlueDEF Diesel Exhaust Fluid is a mixture of high purity synthetic urea and deionized water that is used in Selective Catalytic Reduction (SCR) systems in diesel vehicles. DEF is used to help convert NOx to harmless nitrogen and water. Diesel Exhaust Fluid (DEF) is one of the key elements of the Selective Catalytic Reduction (SCR) process used by Diesel Engine builders to meet EPA 2010 regulations. BlueDEF is a nontoxic solution of 67.5 percent purified water and 32.5 percent ultra-pure automotive-grade urea. BlueDEF is not a fuel or fuel additive. Instead, when injected into the exhaust stream and passed over a catalyst, BlueDEF helps convert NOx into nitrogen gas and water vapor - two harmless and natural components of the air we breathe.