A catalyst unit includes: a ceramic catalyst body through which exhaust gas flows and having a substantially cuboid contour with a substantially rectangular inflow side and a substantially rectangular outflow side; a housing that at least partially encloses the catalyst body, and at least one bearing mat positioned in a gap formed between the catalyst body and the housing. Borders or edges of the catalyst body, which delimit the inflow side and the outflow side, each have dimensions of between 210 mm and 280 mm. The gap between the catalyst body and the housing, seen perpendicularly to the throughflow direction of the catalyst body, has a dimension determined according to the following relationship: s≤p*5, s being the dimension of the gap in mm and p the dimensionless amount of the density of the or each bearing mat expressed in kg/m2.