Diesel Exhaust Fluid (DEF) is one of the key elements of the Selective Catalytic Reduction (SCR) process used by Diesel Engine builders to meet EPA 2010 regulations. BlueDEF(R) is a nontoxic solution of 67.5 percent purified water and 32.5 percent ultra pure automotive grade urea. BlueDEF(R) is not a fuel or fuel additive. Instead, when injected into the exhaust stream and passed over a catalyst, BlueDEF(R) helps convert NOx into nitrogen gas and water vapor - two harmless and natural components of the air we breathe. BlueDEF(R) is stable, colorless and odorless, and meets ISO Standards 22241 for purity and composition and is an American Petroleum Institute (API) certified diesel exhaust fluid. These are the highest quality and safety standards in place to ensure optimum SCR performance. Making sure you do not run out of BlueDEF(R) in a SCR equipped truck is simple - watch the DEF gauge, which is part of the diesel fuel gauge, for when it is time to refill. DEF is stored in a dedicated tank next to the fuel tank on the driver's side. The tanks range in size from 6 to 23 gallons depending on the truck's application.