Abstract: A device for reducing contaminants, particularly for motor vehicles, includes a housing, a catalyst body disposed therein with its periphery spaced from the housing wall and an intumescent mat disposed in the gap between the housing wall and the catalyst body. The intumescent mat is a mat of a heat-resistant fiber mat with embedded expansion particles whose volume increases when a certain expansion temperature is reached that is produced by the hot exhaust gases. By providing an additional heating element in the gap between the housing and the catalyst body, the intumescent mat can be heated to the expansion temperature.