In this study, multivariate examination was used to show the influence of birch extract characteristics on filtration performance in the ultrafiltration based hemicellulose recovery process. Different pretreatments based on adsorption, flocculation or oxidative degradation by enzyme, and a combination of degradation and adsorption, were performed to change the extract characteristics. It was clearly demonstrated that in addition to the total concentration of lignin or carbohydrates in the treated wood extract, also their size has a significant effect on filtration efficiency. Both the ligneous and hemicellulosic compounds, having a negative impact on filtration capacity, seemed to be small enough to enter the membrane skin layer. Thus, in the development of pretreatment processes to enhance filtration efficiency in the recovery of hemicelluloses in biorefineries with ultrafiltration, the focus should not be strictly on maximal removal, but also the size of the lignin molecules has to be considered. From the tested pretreatment methods, the highest increase of flux was achieved with laccase treatment combined with adsorption on bentonite because it decreased the amount of harmful-sized compounds and increased the amount of large compounds, which were favourable to the higher flux. (c) 2015 Elsevier B.V. All rights reserved.