A catalyzer, in particular for internal combustion engines, has a different that widens in the direction of flow upstream of a honeycombed catalyst body (23), a converger (25) that narrows in the direction of flow downstream of the catalyst body (23) and at least one flow guiding body locating within the diffusor and/or converger. In order to achieve a uniform inflow at the front side of the catalyst body (23) without excessively throttling the flow exhaust gases, a flow guiding body (24) composed of a plurality of adjacent and/or imbricated channels having at least partially an increasing cross-section in the direction of flow is arranged at least in the diffusor. The individual channels preferably have an opening angle that prevents burbling at the walls of the individual channels. In addition, the flow guiding body can be coated with a catalytically active material, thus allowing the volume of the diffusor, if necessary of the converger as well, to be also used for housing catalytically active surfaces. This can improve in particular, besides the inflow at the main catalyst body (23), the cold start properties of the catalyzer.