IN order to improve bioethanol production by yeast fermentation of lignocellulosic hydrolysates, sugar/ inhibitor separation by nanofiltration was studied on a bench-scale unit equipped with a spiral-wound membrane. Therefore, a model solution containing 3 sugars and 4 inhibitors was treated with two previously selected membranes (NF270 from DOW Filmtec and DK from GE Osmonics). Both membranes led to high sugar rejection, especially at high permeate flux (>90% for glucose and arabinose and >85% for xylose). Although its water permeability was smaller, DK membrane was preferred for its higher transmission of the inhibitors, especially for the largest ones (vanillin and 5-hydroxymethyl furfural), ensuring a better detoxification level. Diafiltration was applied to improve sugar purity of the treated hydrolysate. With a diavolume equivalent to 1.25 times that of the feed, acetic acid concentration wasdivided by 5 and brought back to concentrations lower than 1 g L-1. A simulation model was proposed to predict the diavolume to apply, depending on the initial concentrations. Finally, processed hydrolysates were tested for the fermentation ability with a Pichia stipitis species. Fermentation tests showed that diafiltration followed by concentration led to retentates as fermentable as an equivalent pure sugars solution.