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Catalytic converters appear to be the most effective means to reduce air pollution from internal combustion engines. The conversion efficiency, however, depends very strongly on the working temperature. The efficiency of a precious metalcontaining catalyst, for example, declines very steeply for a temperature of below around 350°C, and the conversion efficiency is practically zero for cold engines (starting and warming-up period). Preheating of the catalyst with an electrical power, warming it up with an external combustion chamber, and installation of an auxiliary small capacity catalytic converter, are some of the more successful solutions. Although these methods are quite effective, all employ active means that require an external energy source and a control unit. In the present work, the exploitation of thermal capacitance to keep the catalyst temperature high during a short parking period was studied.