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The new stringent regulations for pollutant emissions of handheld power tools such as chainsaws require new solutions to reduce significantly the hydrocarbon emissions. This paper gives an overview of the development work for an exhaust gas aftertreatment system that has to be integrated into a professional chainsaw. The exothermal HC oxidation causes a heat flux of about 1.4 kW for a 50 cm3 chainsaw and therefore the exhaust gas temperature increases from 550°C to 1.000°C after the catalyst within the muffler. On the external side of the engine this additional heat flux is not allowed by the SAE regulation J335 [8] to increase the exhaust gas temperature at the outside surface of the chainsaw above 246°C. This temperature level is nearly reached also without a catalyst, therefore the exhaust gas temperature must be reduced over 750 K, using the same engine design and without changing the outlet direction of the exhaust gas.