Hemicelluloses isolated from agricultural residues have the potential to replace fossil-based materials in high-value-added applications, provided that cost-efficient isolation processes are developed. In this study, a feed solution containing hemicelluloses extracted from wheat bran with a particular high viscosity was used to study the performance of ultrafiltration for hemicellulose isolation at various operating conditions. The operating parameters affected the flux significantly, but had no influence on the retention of the membrane. The flux was doubled by: (i) increasing the cross-flow velocity from 2 to 5 m/s, (ii) increasing the feed temperature from 60 to 80 degrees C and (iii) decreasing the concentration of the solution by half. The flux increase could mainly be attributed to a decrease in the apparent viscosity of the feed solution. This indicates that low feed viscosity is the key to good membrane performance. The limiting flux during ultrafiltration was reached at the low transmembrane pressure of 0.8 bar, even when the cross-flow velocity and feed temperature were as high as 5 m/s and 80 degrees C, respectively. Low transmembrane pressure is thus preferable during ultrafiltration of solutions similar to that used in the present investigation. The results of this study suggest that ultrafiltration could be a suitable method for the isolation of hemicelluloses from high-viscosity solutions. However, as the operating parameters affect the membrane performance significantly, they must be chosen with care to make the process cost-efficient. (C) 2011 Elsevier B.V. All rights reserved.