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. The present system consists of a precious metal-containing catalyst imbedded in a special designed phase-change-material (PCM). Under normal engine operation conditions, some of the thermal energy of the exhaust gases is stored in the PCM in which the catalyst is imbedded. During parking, the PCM undergoes partial solidification and the catalyst temperature is thus maintained inside the desired temperature range for maximal conversion efficiency. The unique features of this method are: a) no special energy source is needed to heat the converter, since the energy of the exhaust gas from the engine is stored and used; and b) the operation is simple and no active or control means are required. The main disadvantage of the method is that it is effective only within a specific length period after engine shut off, the time depending on the thermal specifications of the system.An experimental device on which to conduct feasibility studies was designed, built and tested. It was constructed from a commercially available catalytic converter (with a light-off temperature conversion of about 310°C), in which was embedded 3.8kg of a PCM, composed of a eutectic mixture with additives (353°C melting point and 251.5kJ/kg latent heat of fusion). This design included both an insulation jacket and ceramic pipe fittings to reduce heat losses to the surroundings. The results with this experimental tested system showed that after engine shut-off, the catalyst temperature remained above the highest (for HC) light-off temperature for about 4h. These results demonstrate that this method might be effective in reducing pollutant gases from vehicles in large cities, where the average number of re-startings per vehicle per day is high.