Process and waste waters of pulp and paper mills contain valuable and underexploited compounds. Interest in utilizing such compounds has increased, leading to the study and development of forest-based biorefineries. Efficient separation technology is vital for the economics of biorefineries. Recovery of valuable compounds, such as lignin or hemicelluloses, can be accomplished with membrane processes. In principle, ultrafiltration is suited for the recovery, fractionation and purification of hemicelluloses from wood hydrolysates. Unfortunately, wood hydrolysates have a high fouling tendency which might lead to inefficient process operations due to decreased filtration capacity and increased costs. This study evaluates several pretreatment methods for hydrolysates prior to ultrafiltration. The pretreatment methods were selected based on the most probable foulants in wood hydrolysates, striving for their removal, degradation or inactivation. The efficiency of the pretreatment methods in fouling prevention was evaluated by filtering various wood-based hydrolysates with two commercial ultrafiltration membranes, a polysulphone (PSu) membrane and a regenerated cellulose (RC) membrane. The hydrophobic PSu membrane was fouled more in the experiments than the hydrophilic RC membrane. In tests with the PSu membrane, the most significant improvement in filterability was achieved when pulsed corona discharge (PCD) treatment, an oxidation method, was used as pretreatment. The PCD treatment, which was tested only for the PSu membrane, increased the flux over sixfold compared to filtration of untreated spruce hydrolysate. Other pretreatment methods, such as activated carbon adsorption and pH adjustment, which were tested with both membranes, also had positive effects on the filtration capacity. The results of this study indicate that pretreatment processes have to be tailored separately for different hydrolysates due to their different characteristics. (C) 2011 Elsevier B.V. All rights reserved.