An aqueous urea solution is used as the source of ammonia for selective catalytic reduction (SCR) of NOx to reduce the emissions of NOx in the exhaust of diesel vehicles. However, the decomposition of urea into ammonia is not always complete, resulting in solid urea deposit formation in the decomposition tube or on the SCR catalyst. These solid deposits can impede the flow of the exhaust gases (and uniformity of NH3 supply) and reduce SCR catalyst performance over time. To minimize the formation of urea deposit and to meet EPA NOx emission regulations, it is important to understand the chemistry of formation or removal of the deposit in the decomposition tube and SCR catalyst. In this report, IR spectroscopy, UV-visible spectroscopy, thermogravimetric analysis and elemental analysis have been used to determine the chemical composition of the solid urea deposits formed by the thermal decomposition of urea. Small and bulk urea deposit samples that formed during engine dyno tests with exhaust temperatures between 200 and 425 °C were analyzed. The results showed that exhaust temperatures of 200-250 °C inside the decomposition tube yielded unreacted urea, biuret and cyanuric acid. However their proportions varied appreciably from test to test. At exhaust temperatures of 335-425 °C, the deposits consist of mostly cyanuric acid. Small scale tests results revealed that urea, biuret and cyanuric acid in the solid urea deposits were not completely removed even though the exhaust temperature exceeded the temperatures at which these compounds thermally decompose.