A hydrocyclone chamber includes a cylindrical part having a tangential inlet and a central outlet for a separated fraction of the incoming mixture, the chamber also including a conical part leading to a second outlet for another fraction of the mixture. A guide bar extends circumferentially along the wall of the cylindrical part at least to the inlet orifice and is bent and inclined to give the mixture flowing from said inlet a component of movement directed radially inward and a component of movement directed axially toward said conical part of the chamber.