To remove fermentation-inhibiting compounds and increase sugar concentrations of paulownia wood extracts, the extracts were passed through a set of nanofiltration membrane. During the process, the changes of the concentrations in both retentate and permeate flows were determined. For individual components, a simplistic model based on mass balance was established for the process. The results show that most of the fermentation inhibitants, such as furfural, hydroxymethylfufural, free acetic acid, formic acid and methanol are removed while most of the desirable components, i.e. sugars, are preserved. Nanofiltration is found to be an efficient way to purify and concentrate wood extracts. (C) 2015 Elsevier B.V. All rights reserved.