Abstract: Silencer with built-in catalyser which utilises a given total space optimally for simultaneous sound attenuation and catalytic treatment of gases, e.g. exhaust gases from internal combustion engine. The silencer with built-in catalyser comprises a casing (1) connected to an inlet pipe (2) and to an outlet pipe (3), a sound attenuation compartment (4), a downstream catalytic body (5), a flow-area widening diffuser element (6) extending from the inlet pipe and contained within the compartment, and a cross-plate (7) which is positioned between the diffuser element and the catalytic body and from which the flow is distributed evenly across the inlet to the catalytic body. At least two openings (8) are provided between the diffuser element and the catalytic body, at least one such opening (9) providing a communication to the sound attenuation compartment and at least two such openings (10) being pervaded by gas flows.