The cold-start of a prototype diesel engine exhaust aftertreatment system was analyzed using a simplified energy balance to study the impact of system design changes on the performance of the selective catalytic NOx reduction reactor. The simplified energy balance method is shown to be a viable tool for system-level analysis of the aftertreatment performance. The results indicate that without an external energy supply the best way to shorten the selective catalytic reduction (SCR) reactor light-off time is to reduce the system thermal inertia by including a metallic diesel oxidation catalyst (DOC) and moving the SCR reactor upstream. Such optimization of the aftertreatment architecture is found to significantly reduce SCR light-off time for the configurations examined. Electrical heating applied to the SCR and DOC reactors can also reduce the light-off time. The system architecture optimization, however, is subject to vehicle under-hood packaging restrictions. To meet more stringent emission standards in the future, a combination of architecture optimization and electrical heating will be required. (C) 2009 Elsevier B.V. All rights reserved.