Lignocellulosic biomass is a renewable feedstock for the production of fuels and chemicals. After acid pretreatment and enzymatic hydrolysis, the biomass produces a stream containing dilute sugars, along unwanted fermentation inhibitory compounds. Two promising approaches to increase process efficiencies and economics are to increase the sugar concentration and decrease inhibitor concentrations prior to fermentation. In this study, reverse osmosis (RO) and nanofiltration (NF) membranes were evaluated for their ability to separate inhibitors (e.g., organic and mineral acids, furans and phenolic compounds) from sugars, while simultaneously concentrating the sugars. A range of RD and NF membranes were tested that exhibited different chemistries and pore-sizes, and performance evaluation was assessed by measuring permeate flux, retention (yield) of sugars, the ability to separate inhibitors, and ease of cleaning. Performance varied considerably between the different membranes evaluated, with sugar yields in the retentate ranging from below 20% to nearly 100%. Generally, lower sugar yields also corresponded to higher inhibitor separation factors. Flux reduction for most membranes was primarily caused by reversible fouling, but in many cases a significant contribution also came from elevated osmotic pressure due to retention of soluble compounds. To improve fouling characteristics and maintain elevated flux during operation of the RD and NF membranes, lower surface roughens and higher hydrophilicity provided more favorable properties, along with larger number of pores. At the same time, the ability to separate inhibitors, which contributed significantly to osmotic pressure flux reduction, was also beneficial. (C) 2013 Elsevier B.V. All rights reserved.