The invention relates to a method for concentrating a material solution using at least two consecutive membrane separating stages each for separating the material solution into a retentate and a permeate the cut-off of the membranes used in the separating stages being higher than that of the membrane used in the previous stage the retentate of each separating stage with the exception of the last stage being fed to the following separating stage as a feed and the permeate from at least one separating stage being fed back to a preceding separating stage and introduced into the feed thereof characterized in that the fed back permeate has a concentration and viscosity that substantially corresponds to the concentration and viscosity of the preceding separating stage into which said permeate is introduced. (Fig. 2)
