This work is devoted to the separation of polyphenols and proteins, extracted from flaxseed hulls. The separation comprised two steps: (i) coagulation of proteins by adjustment of pH and subsequent centrifugation, and (ii) ultrafiltration of supernatants. The value of pH, used for extract coagulation, varied from 13.2 to 3.0. Ultrafiltration experiments were carried out in the concentration mode (up to the final volume reduction factor of 7.5) using a batch dead-end stirred filtration cell and hydrophilic polyethersulfone membranes with nominal molecular weight cut-off 30 kDa. The impact of pH on the efficiency of protein coagulation and filtration steps of polyphenol purification was investigated and optimal conditions of purification were determined. The purity of polyphenols in filtrate increased from 33.6% to 56.0% after coagulation and centrifugation at pH = 4.4, and it was additionally increased to 76.6% after the ultrafiltration. The impact of pH on filtration rate decline and rejection of polyphenols and proteins was demonstrated. Filtration rate decreased and filtrate purity increased as pH decreased from 13.2 to 4.4. The decrease of filtration rate at lower values of pH was explained by formation of a less permeable filter cake of residual proteins. It was demonstrated that filter cake formation significantly increased the selectivity and resulted in a better filtrate quality. (C) 2013 Elsevier B.V. All rights reserved.