A mixer / evaporator or an exhaust system (5) of an internal combustion engine (1), includes a carrier (20) surrounding, in a circumferential direction, a flat cross section of a mixing / evaporating device (12), through which exhaust gases flow and which extends at a right angle to the axial direction (17). The carrier has opposite side walls, wherein short side walls (23, 24) connect each long side walls (21, 22) to one another. Guide blades (25) project in a direction of the respective opposite long side wall, are set at an angle in relation to the axial direction and are arranged at one of the long side walls at least one axial end (26, 27). An axial end of the first long side walls is offset in the axial direction in relation to the axial end of the second long side wall on the same side of the mixing / evaporating device.