Abstract: An apparatus for silencing and catalytic treatment of gases has an air-tight casing connected to an exhaust inlet pipe and to an exhaust outlet pipe, acoustic compartments, monolithic bodies, and a diffuser element. The diffuser element is connected to the inlet pipe, or further downstream within the casing. The diffuser element distributes the gases evenly across the inlet face of the monolithic bodies. The diffuser element is a guide baffle or plate and a juxtaposed stagnation baffle or plate, which causes both flow stagnation in front of the stagnation plate and the gases to flow radially within the diffuser element. The diffuser element has apertures which are pervaded by partial flows of the gas and are adapted to provide additional recovery in the gas flow passing through the diffuser element. The geometry defining the fluid flow field within the diffuser is designed to prevent flow separation from the contour walls of the diffuser.