Abstract: Unbleached sulphae fibres are chemically separated from other fibres by the use of high molecular polyalkylene oxides, preferably polyethylene oxide. The polyalkylene oxides are added to aqueous suspensions of fibre mixtures and hereby flocculates unbleached sulphate fibres in a selective manner and the flocks can be separated by known methods such as sedimentation, flotation, filtration etc. Use of polyalkylene oxides for selective flocculation of unbleached sulphate fibres gives the possibility of upgrading for example recovered fibres, broke and secondary fibre raw materials.