The modeling of transient phenomena occurring inside an automotive 3-way catalytic converter poses a significant challenge to the emissions control engineer. Since the significant progress that has been observed with steady-state models cannot be directly exploited in this direction, it is necessary to develop a fully transient model and computer code incorporating dynamic behaviour of the three way catalytic converter in a relatively simple and effective way. The Laboratory of Applied Thermodynamics (LAT), Aristotle University Thessaloniki, is cooperating with the Engine Direction of FIAT Research Center, in the development of a computer code fulfilling these objectives, within the framework of an EEC Brite EuRam cost shared project. The CRF and LAT modeling approaches, along with the underlying philosophy and experimental work, are presented in this paper. The model and computer codes developed are currently employed to establish and understand critical behaviour trends involved in engine emission control system optimization.