# Diesel Exhaust Fluid: What DEF Is, What It Is Made Of and How It Works Modern diesel engines are powerful, efficient, and essential to transportation, agriculture, construction, and many other industries. They must also control the nitrogen oxide emissions produced during combustion. That is where Diesel Exhaust Fluid plays an important role. **Diesel Exhaust Fluid, commonly called DEF, is a clear solution used in diesel vehicles and equipment with selective catalytic reduction systems. It contains 32.5% high-purity urea and 67.5% purified water. Injected into hot exhaust—not the fuel tank—it...