Selective catalytic reduction (SCR) technology is capable to deliver significant reduction of NOx emissions from diesel engines. This system relies on the use of urea as a reductant that is sprayed into the exhaust flow before reaching the ceramic substrate containing the SCR catalyst. This study describes the effect of urea exposure on substrate support mats when installed in full scale SCR units. Individual systems were aged to 500, 1000, 2000 and 3500 hours. After the aging was completed the devices were torn down and testing was completed on the mat and substrate. Tests were performed to search for the presence of crystalline urea inside the support mat fiber matrix and several analytical tests were performed to determine whether urea exposure and built up caused deterioration of mat pressure and its long term durability.