Understanding urea decomposition is critical to achieve highly efficient selective catalytic reduction (SCR). The urea decomposition process in an exhaust pipe and in Cu-zeolite and vanadia-SCR (V-SCR) was studied in engine test cells. The unconverted urea at the SCR inlet can be substantial at lower temperatures. HNCO and NH(3)are two dominant products at the SCR inlet. Urea and HNCO continue to decompose in SCR catalysts, with a rate much faster than in the homogeneous stream. The HNCO hydrolysis process is extremely efficient in Cu-zeolite SCR and the abundant NH(3)from urea overdosing can improve the NO(x)conversion efficiency. While for V-SCR, the HNCO hydrolysis reaction can become the rate-limiting step (especially after aging), abundant urea at low temperatures impairs NO(x)reduction.