Lignocellulose has drawn great attention in the bioethanol industry as an alternative feedstock for ethanol production due to its renewability, abundance and non-food crop characteristics. Acid hydrolyzation of lignocellulose releases sugars (mainly D-Xylose) and several derivatives. The sugars in the hydrolyzate are then converted into ethanol by fermentation. Since acetic acid is believed to be one of the inhibitors which limit the yield of ethanol, it is beneficial to remove acetic acid from the hydrolyzates before fermentation. In this study, a Desal-5 DK nanofiltration (NF) membrane was used to separate acetic acid from xylose, using a synthetic acetic acid-xylose solution as the model. The Desal-5 DK membrane had an isoelectric point of 3.7 and a pore size of 0.83 nm based on streaming potential measurement and model calculation. It was found that both the solution pH and the applied pressure affected the separation performance. The observed retention of xylose and acetic acid varied from 28% to 81% and -6.8% to 90%, respectively, depending on the solution pH and the applied pressure. The maximum separation factor was 5.4 when the system was operated at pH 2.9 and 24.5 bar. In addition, negative retention of acetic acid was observed only in the presence of xylose. The results suggested that intermolecular interactions play an important role in the separation of xylose and acetic acid. (C) 2009 Elsevier B.V. All rights reserved.