The focus of this study was to find out the effect of bark on SO2-ethanol-water (SEW) fractionation and enzymatic hydrolysis of forest biomass. Softwood bark was found to be more harmful than hardwood bark in both processes. For softwood, the amount of undigested wood in SEW fractionation increased with the increasing bark content, whereas the hardwood bark did not impair the fractionation of wood. The higher the softwood bark content was the lower were the yields in enzymatic hydrolysis likely due to the unproductive binding of enzymes on lignin and other compounds. Addition of surfactant Tween 20 (2% w/w on substrate) prior to enzyme more than doubled the sugar yield of bark-rich softwood pulp. Hardwood bark impaired enzymatic hydrolysis when its share was over 28%. According to a preliminary study, lignosulfonates from the carry-over liquor seem to improve the sugar yield in the enzymatic hydrolysis by acting as a surfactant. (C) 2014 Elsevier Ltd. All rights reserved.