The performance of selective catalytic reduction monolithic catalysts may best be characterized by plotting the ammonia slip as a function of the percentage of NO, conversion achieved. Experimental results obtained on a Diesel engine test stand have been compared with predicted values obtained by model calculations. In this way the possibilities and Limits of such calculations could be shown. The model has no adjustable parameters. An exact prediction of performance is not possible under all conditions due to minor differences in composition between the real exhaust and the test gas mixtures used in the determination of the intrinsic catalyst properties. The value of such a model therefore lies in the prediction of the performance of new catalysts (e.g., layer catalysts) or under changed operating conditions (other temperature or GHSV).