Automotive exhaust emission regulations are becoming progressively stricter due to increasing awareness of the hazardous effects of exhaust emissions. The main challenge to meet the regulations is to reduce the emissions during cold starts, because catalytic converters are ineffective until they reach a light-off temperature. It has been found that 50% to 80% of the regulated hydrocarbon and carbon monoxide emissions are emitted from the automotive tailpipe during the cold starts. Therefore, understanding the catalytic converter characteristics during the cold starts is important for the improvement of the cold start performancesThis paper describes a mathematical model that simulates transient performances of catalytic converters. The model considers the effect of heat transfer and catalyst chemical reactions as exhaust gases flow through the catalyst. The heat transfer model includes the heat loss by conduction and convection. A 13-step reaction mechanism is used to simulate the chemical kinetics, and a 9-step reaction scheme is used to simulate oxygen storage mechanism, which allows adsorption/desorption of oxygen in the catalyst during the flow of non-stoichiometric air-fuel mixtures. This model is used to predict the catalytic converter performances during cold starts. The computational results of emission conversions for different initial converter temperatures are found to be in fair agreements with experimental measurements. This model is also used to study the catalytic converter light-off characteristics during the US Federal Test Procedure (FTP). It is found that the light-off time can be reduced to zero if the converter is heated to 600 K before the FTP test running.