A hydrocyclone is disclosed for separating, at least partially, fluid mixtures having a predominant liquid component. The hydrocyclone has a first end and a second end remote from the first end. The cross-sectional area of the hydrocyclone in at least one location towards the second end is less than the cross-sectional area at the first end. The hydrocyclone further includes at least one inlet in the region of the first end for introducing the feed mixture and at least two outlets with at least one outlet in the region of the second end. The hydrocyclone has in the region of the second end a fixed or moveable flow-modifying element located at or near the hydrocyclone axis and the element is constructed to affect the flow towards the second end of fluid containing a relatively large portion of less dense component but to allow flow past the element towards the second end of the fluid containing a relatively large portion of a more dense component. Preferred geometric and flow parameters for the cyclone are disclosed.