$4.95 each Diesel Exhaust Fluid (DEF) is an ultra-pure urea solution that acts as a reductant. It is carried in a separate tank, and is then injected into the hot exhaust stream and absorbed onto a catalyst. It is of 32.5% high-purity urea and 67.5% deionized water. The purpose of DEF is to reduce Nox concentration in the exhaust emissions produced by diesel engines. The process by with the NOx emissions are reduced is called Selective Catalytic Reduction (SCR).