Manufacturers of automotive catalytic converters are constrained to design more effective and reliable systems to meet the stringent emission limits anticipated for the end of the decade. Reactor models and computer simulations may offer new possibilities for optimal catalytic converter design. For this reason, transient models have been developed using the commercial CFD (Computer Fluid Dynamics) code PHOENICS.In this work the effects of catalyst design parameters are studied using the one-dimensional transient model for a single catalyst channel. The results indicate that the catalyst precious metal loading has a great influence on the catalyst light-off, and the geometric area (catalyst length and cell density) and the hydraulic radius (catalyst cell density) on the steady state conversion efficiency. The ceramic catalyst seems to have a higher tendency towards thermal shock than the metallic. The optimization of the metallic catalyst behavior inevitably leads to the use of a high cell density to obtain the best steady state behavior. The precious metal loading needed to reach fast light-off can be optimized by shortening the substrate length.The behavior of the advanced metallic catalyst is compared with the 400 cpsi ceramic catalyst using the three-dimensional transient model. This simulates the fluid dynamics, chemical kinetics and heat and mass transfer taking place in a catalyst and its assembly. The volume of the metallic catalyst is a half of that of the ceramic. The geometric areas of the catalysts are almost identical. Both catalysts contain the same amount of precious metals. The length of the ceramic honeycomb is double compared to that of the metallic. Due to the smaller heat capacity, better heat conduction and heat transfer the 800 cpsi metallic catalyst heats up faster than the 400 cpsi ceramic one. Furthermore, the better mass transfer characteristics of the metallic catalyst enable an almost 4.5 % higher conversion efficiency than the ceramic.