This paper presents a mean-value model describing the SCR system of a heavy duty mobile application. The system consists of two catalysts, one for the preoxidation and one for the selective catalytic reduction (SCR) of NOx. The model of the catalytic converter system is one-dimensional. Its core consists of three identical cells that describe the thermal and the chemical behavior of the SCR catalyst. The performance of the model is verified by comparing results of simulations and of measurements taken on a dynamic test bench.