Degradation of the deNOx performance has been found in in-use heavy-duty vehicles with a urea-SCR system in Japan. The causes of the degradation were studied, and two major reasons are suggested here: HC poisoning and deactivation of pre-oxidation catalysts. Hydrocarbons that accumulated on the catalysts inhibited the catalysis. Although they were easily removed by a simple heat treatment, the treatment could only partially recover the original catalytic performance for the deNOx reaction. The unrecovered catalytic activity was found to result from the decrease in conversion of NO to NO2 on the pre-oxidation catalyst. The pre-oxidation catalyst was thus studied in detail by various techniques to reveal the causes of the degradation: Exhaust emission tests for in-use vehicles, effect of heat treatment on the urea-SCR systems, structural changes and chemical changes in active components during the deactivation were systematically investigated. Laboratory experiments to reproduce the degradation using simulant exhaust gases and catalyst samples were also conducted. Aluminum sulfate was indicated to be a major cause of the deactivation. It was produced mainly at approximately 300°C through oxidation of SO2 to SO3, reaction of SO3 with water, and finally reaction of Al2O3 with H2SO4 to generate Al2(SO4)3. The effects of aluminum sulfate on the activity of oxidation catalysts remained unknown.