The thermal and catalytic decomposition of urea over a fixed-bed flow reactor system has been examined for the selective catalytic reduction (SCR) of NO x from mobile sources. The conversion of urea into NH3 and HNCO, the two major products from the thermal decomposition of urea, increased with the reaction temperature and the reactor space time. Urea was completely decomposed into NH3 and HNCO at 350 °C when the residence time was longer than 0.1 s. As the reaction temperature increased to 400 °C or higher, complete decomposition of urea was possible at a much shorter residence time of the feed gas stream. The simultaneous thermal and catalytic decomposition of urea was also examined in a dual-reactor system in which the first reactor was for thermal decomposition and the second was for catalytic decomposition, specifically over copper exchanged ZSM5 catalyst. The role of the catalyst in the decomposition of urea into NH3 and HNCO was negligible; urea decomposition occurs mainly by the thermal reaction. However, the catalyst was able to rapidly hydrolyze HNCO to NH3 even at a temperature as low as 150 °C. The catalyst also oxidizes NH3 to N2 in the presence of oxygen at reaction temperatures above 250 °C. A kinetic model describing all of the major reactions involved in urea decomposition over an SCR reactor has been developed. The model adequately predicts the general trend of variations of the urea, NH3, and HNCO concentrations with the reaction conditions.