Lignocellulosic materials can be acid-hydrolyzed into fermentable sugars for ethanol production. However, inhibitory compounds such as carboxylic acid, furans and phenolic substances, are also generated in this process. It is necessary to separate these inhibitors from sugars or reduce the concentration of the inhibitors prior to the fermentation process. Our work focused on the separation of acetic acid from monosaccharides, due to its high concentration and strong toxicity for subsequent fermentation. The separation performance of acetic acid from monosaccharides by NF/RO membranes was compared to evaluate the feasibility of simultaneous acetic acid separation and sugars concentration, by using synthetic xylose-glucose-acetic acid model solution. The influences of operation conditions such as pH, pressure, temperature and feed concentration on the retention of solutes were investigated, while the stimulation effect of acetic acid at low concentration on fermentation process was also discussed. Consequently, extremely high separation factor of acetic acid over monosaccharides of a RO membrane RO98pHt was reported, and the separation factor of this membrane is significantly greater than the separation factor of NF membranes that have been considered as the standard membranes for this application so far. Considering that the sugars should be retained as much as possible during detoxification process, and the low concentration of acetic acid has a promoting contribution to ethanol production, RO98pHt is the best membrane for simultaneous acetic acid separation and sugar concentration. Therefore, we suggest that RO process instead of NF process should be used for this application. (C) 2012 Elsevier B.V. All rights reserved.