Heat transfer study is particularly important in a modern engine exhaust system incorporated with a catalytic converter. Heat loss along the exhaust system, which includes exhaust ports, manifold, tailpipe and catalytic converter, causes prolonged lightoff time of catalysts at engine cold-start. This implicitly means that more noxious gas emissions in the cold-start phase will be produced at the engine tailpipe, which contributes to an overall increase in exhaust toxic emissions, in particular for urban driving where heavy traffic is frequently encountered. Modeling of exhaust heat transfer is thus necessary, as it is a powerful and cost effective tool for estimating the lightoff time and conversion efficiency of the catalysts. Furthermore, the concentrations of noxious gases in the exhaust tailpipe can be calculated.This paper considers the chemical conversion of carbon monoxide and unburned hydrocarbons in the oxidation process with detailed heat transfer modeling, which includes conductive, convective and radiative heat transfers. The effects of water vapor condensation and water evaporation on temporal distribution of exhaust gas temperature in the exhaust system are also considered with simplified heat transfer model.