The invention relates to a method and apparatus for thickening liquids carrying suspended particles, wherein a feed slurry is fed to a thickener tank (2) via a feedwell (4). The problem to be solved is to reduce loss of mixing in the feedwell caused by variations in the flowrate of the feed slurry. The solution is to vary the cross-sectional area of the feedpipe (8) in response to variations in the flowrate inside the feedpipe, in order to maintain a sufficient velocity for slurry exiting the feedpipe (8).