With the increasingly widespread use of catalytic converters for meeting exhaust emission regulations, considerable attention is currently being directed towards improving their performance. Experimental analysis is costly and time consuming. A desirable alternative would be a computational model based on established numerical techniques. To this end a transient three-dimensional model has been developed using a commercial CFD code. It simulates the fluid dynamics, chemical kinetics and heat and mass transfer that takes place in catalysts and their associated assembly. As a result the model can be used to predict important performance parameters such as conversion efficiency, incurred pressure drop and the thermal environment.