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Diesel exhaust fluid (DEF) is an aqueous urea solution made with 32.5% urea and 67.5% deionized water. It is standardized as AUS 32 (aqueous urea solution) in ISO 22241.^[3] DEF is used as a consumable in selective catalytic reduction (SCR) in order to lower NO_x concentration in the diesel exhaust emissions from diesel engines

SCR systems are sensitive to potential chemical impurities in the urea solution, therefore the solvent is demineralized water. The urea solution is clear, non-toxic and safe to handle. However, it can corrode some metals and so must be stored and transported carefully.

DEF is stored in a tank on board the vehicle, and injected into the exhaust stream by a metering system. The injection rate depends on the specific after-treatment system, but is typically 2–۶% of diesel consumption volume. This low dosing rate ensures long fluid refill intervals and minimizes the tank’s size (and subsequent 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 exhaust fluid 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. As of 2013, a number of truck stops are beginning to add DEF pumps, in which diesel exhaust fluid is administered at pumps similarly to diesel, and often located adjacent to fuel pumps so that the vehicle operator can fill up on both without moving the truck.