Urea decomposition in the urea selective catalytic reduction (urea-SCR) system is achieved under a multiphase flow field coexisting with the injected urea solution droplets in the liquid phase, the inflow gas in the gas phase, and solid urea particles. For this reason, urea decomposition is closely connected with thermo-fluid dynamics as well as various chemical reactions. An experimental study was performed to investigate urea decomposition under various thermo-fluid conditions, with different temperatures and velocities of inflow gas, and urea solution quantities. The inflow gas conditions were similar to the exhaust conditions of a large marine engine. A urea solution injector of twin fluid type was designed, as well as a lab-scale exhaust system to control the temperature and velocity of the inflow gas. The spray performance of the designed urea solution injector was almost identical under all the experimental conditions used, so the spray's effect on the urea decomposition was minimized. The total conversion efficiency considered with both NH3 and HNCO was closely related to the urea injection quantity and convective heat transfer of the inflow gas as well as the temperature of the inflow gas. The conversion efficiency of NH3 was larger than that of HNCO under all experimental conditions, unlike for the theoretical thermolysis reaction. The difference between the conversion efficiency of NH3 and HNCO was affected by convective heat transfer and urea injection quantity as well as the temperature of the inflow gas. (C) 2014 Elsevier B.V. All rights reserved.