Diesel Exhaust Fluid (AdBlue) is a consumable fluid for the Selective Catalytic Reduction (SCR) exhaust aftertreatment system on diesel engines.

It is a high-purity 32.5% aqueous urea solution that is metered into the exhaust stream upstream of the SCR catalyst, where it reacts with oxides of nitrogen (NOx) and converts them into nitrogen and water to help the engine meet diesel emissions standards. It is held in the vehicle’s dedicated DEF tank and replenished periodically.