DEF is a special urea solution that is used as a reducing agent in SCR (Selective Catalytic Reduction) technology. DEF is a high-purity 32.5% aqueous urea (ammonia) and 67.5% de-ionized water for specific use in SCR systems.
Diesel-powered vehicles with an SCR catalytic converter and an additional separate tank for DEF can convert their output of nitrogen oxides into neutral nitrogen and water, thus achieving a considerable reduction of their NOx emissions.