Diesel exhaust fluid (DEF) is a blended aqueous solution composed of 32.5% high-purity urea and 67.5% deionized water. It acts as a reactant for the functioning of the selective catalytic reduction (SCR) system, an advanced active emission control technology system. DEF is standardized as aqueous urea solution (AUS) 32 in ISO 22241. AUS 32 is registered as a trademark AdBlue by the German Association of the Automotive Industry (VDA).