Abstract: A monolithic type of catalyst carrier for the exhaust system of a combustion engine is securely held in a tubular metal housing under the variations in temperature of operation by surrounding the monolith with a jacket, which extends between the monolith and the casing, and which has a thermal expansion coefficient that is equal to or greater than the difference between the thermal expansion coefficient of the monolith and that of the metal of the casing. The tubular casing also has tubular conduits on both ends with gradually decreasing diameters as the distance from the casing increases. Packing rings are provided between the peripheral edges of the monolith and the ends of the connected tubular conduits. Preferably, also, the end peripheral edges of the monolith are further protected by a flanged metal ring of which the flanged portions extend over a portion of the periphery at the ends of the monolith.