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In this investigation steam pretreatment of willow with the addition of SO2 or H2SO4 was studied, the aim being to recover both the cellulose and the hemicellulose fr actions. The willow was steamed (1 bar saturated steam) prior to impregnation with 0-3% H2SO4 or 1% SO2 (w/w substrate). The pretreatment experiments were performed at temperatures in the range 160-230 degrees C for 10 min. The highest xylose recovery with H2SO4 impregnation was obtained using 3% H2SO4 at 190 degrees C, where 80% of the available xylose could be recovered by washing. At these pretreatment conditions, a glucose yield after enzymatic hydrolysis of 67%, based on the glucan available in the raw material, was achieved. The maximum glucose yield, 95%, was obtained when the willow was impregnated with 1% SO2 and pretreated at 200 degrees C. At these conditions, 62% of the original xylose was solubilized. As glucan is the main constituent of willow, a high yield of glucose is of more importance than a high yield of xylose, leading to the conclusion that impregnation with SO2 is better than impregnation with H2SO4.