Operational conditions for the hydrolysis of rape straw were optimized using the combined severity index (CS), which combines the effects of time, temperature, and acid concentration into a single parameter. The sugar recovery yield was 77.8% of the theoretical yield at a value of CS = 1.3. A maximum concentration of xylose of 7.22 g/L was obtained when the straw was treated for 10 min at a low reaction temperature (150 degrees C) and high acid concentration (pH 1.17). The pentose-rich hydrolyzate exhibited a low concentration of fermentation-inhibiting compounds. The concept of CS can be conveniently and effectively applied for optimization of pretreatments. (C) 2011 Elsevier Ltd. All rights reserved.