'''Diesel Exhaust Fluid''' ('''DEF'''), commonly referred to as '''AdBlue''' or '''Ad-Blue''' and standardised as '''ISO 22241'''<ref name="ISO">{{cite web|title = ISO 22241 Specification|url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=42941|publisher=ISO|accessdate=15 March 2012}}</ref> is an '''Aqueous Urea Solution''' made with 32.5% high-purity [[urea]] ('''AUS 32''') and 67.5% [[deionized water]]. DEF is used as a consumable in [[Selectiveselective Catalyticcatalytic Reductionreduction]] (SCR) in order to lower [[NOx|NO_x]] concentration in the [[Dieseldiesel exhaust]] emissions from [[Dieseldiesel engine]]s.<ref name="CumminsFiltration">{{cite web|url=http://www.cumminsfiltration.com/pdfs/product_lit/americas_brochures/MB10033.pdf|work=Cummins Filtration|title=What is DEF?}}</ref>

Diesel engines run with a [[lean burn]] air-to-fuel ratio ([[Stoichiometry#Stoichiometry of combustion|overstoichiometric]] ratio) to ensure the full combustion of soot and prevent unburnt fuel from being exhausted. The excess of oxygen necessarily leads to generation of [[NOx|nitrogen oxides]] (NO_x), which are harmful [[pollutant]]s, from the nitrogen in the air. Selective catalytic reduction is used to reduce the amount of NO_x released into the atmosphere. Diesel exhaust fluid (from a separate DEF tank) is injected into the exhaust pipeline, the aqueous urea vaporizes and decomposes to form [[ammonia]] and [[carbon dioxide]]. Within the SCR [[catalyst]], the NO_x are catalytically reduced by the ammonia ([[Ammonia|NH₃]]) into [[water]] (H₂O) and [[nitrogen]] (N₂), which are both harmless; and these are then released through the exhaust.<ref name="H2Blu">{{cite web|url=http://h2blu.ca/about-def/how-it-works|work=H2Blu|title=How it works}}</ref>

DEF is stored in a tank on board the vehicle, and injected into the exhaust stream by a metering system at a rate of 2% of Dieseldiesel consumption volume (4–6% within Europe). This low dosing rate ensures long fluid refill intervals and minimises the tank's obtrusion into vehicle packaging space. An [[electronic control unit]] adjusts the addition of fluid in accordance with such parameters as engine [[operating temperature]] and speed.

Diesel Exhaustexhaust Fluidfluid is offered to consumers through a variety of quantities by manufacturers ranging from containers of it for single or repeated small usage, up to bulk carriers for consumers that require a large amount of DEF. It is recommended that DEF be stored in a cool, dry, and well-ventilated area that is out of direct sunlight.

In the UK the storage of DEF is now under scrutiny of the Environment Agency who are proposing new controls following an number of spills of DEF into the environment. Although not toxic, DEF can cause the catastrophic de-oxidation of water bodies leading to damage of the aquatic environment.<ref>{{brokenlink}}{{cite web|url=http://www.environment-agency.gov.uk/business/topics/pollution/39083.aspx|title=Large spills into water bodies}}</ref>