A device for the treatment of exhaust gases leaving an internal combustion engine in which a main flow passage for the gases is surrounded by another passage having therein axially arranged treatment chambers containing catalyst material. The main flow passage is provided with a flow control valve which, at low engine speeds, diverts substantially all of the exhaust gases to the outer passage surrounding the main flow passage, while at higher speeds the valve deflects and permits an ever increasing amount of the exhaust gases to pass through the main flow passage. The main flow passage and the outlet end of the outer passage containing the catalyst chambers are coaxially arranged and taper inwardly so that there is a mingling of the gases from the passages at the discharge end of the device. The device is adapted for direct connection to the exhaust manifold of an engine and can be incorporated directly therein.