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. Diesel Exhaust Fluid is a nontoxic solution of 67.5% purified water and 32.5% ultra pure automotive grade urea. Diesel Exhaust Fluid is not a fuel or fuel additive. Instead, when injected into the exhaust stream and passed over a catalyst, Diesel Exhaust Fluid helps convert NOx into nitrogen gas and water vapor – two harmless and natural components of the air we breathe.