Hemicelluloses could be used to replace fossil-based materials in several high-value-added products. Today, vast amounts of hemicelluloses are discharged from pulp mills all over the world as waste, but these could be isolated by membrane filtration, and utilized in various applications. In this study, the hemicellulose galactoglucomannan was isolated from process water from a thermomechanical pulp mill using ultrafiltration. The retention of hemicelluloses and lignin, and the flux and fouling of three ultrafiltration membranes (ETNA01, ETNA10 and UFX5) were studied at various operating conditions. One of the membranes (UFX5) was found to have a high hemicellulose retention (above 90%) independent of flux and pressure. With the ETNA01 membrane it was impossible to combine a high flux with high hemicellulose retention, while with the ETNA10 membrane the hemicellulose retention could be increased above 90% by running at transmembrane pressures above the critical flux. The UFX5 membrane could be used at the temperature of the process water in the pulp mill (75-85 degrees C), while the ETNA10 membrane could only withstand temperatures below 60 degrees C, increasing the cost due to the need to cool the process water. However, the susceptibility of UFX5 to fouling was much greater than for ETNA10, which would increase the cleaning cost of the UFX5 membrane. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.