In a hydrocyclone plant for separating a fiber pulp suspension into a light phase and a heavy phase, comprising at least two hydrocyclones (1, 2) connected in series, there is provided a device (20) arranged both for degassing a first stream of separated heavy phase coming from one of said hydrocyclones (1) and a second stream of liquid supplied to dilute the first stream, and for mixing said two streams before they are supplied to the other one of said hydrocyclones (2).