Diesel Exhaust Fluid (DEF) is one of the key elements of the Selective Catalytic Reduction (SCR) process. It is used by Diesel Engine vehicles to meet emissions requirements. DEF is a nontoxic solution of 67.5% purified water and 32.5% ultra pure automotive grade urea. DEF is not a fuel or fuel additive. Instead, when it is injected into the exhaust stream and passed over a catalyst, it helps convert NOx into nitrogen gas and water vapor – two harmless and natural components of the air that we breathe.