The production of fermentable substrates from barley straw under various process conditions was studied. Pretreatment included chemical pretreatment with dilute-acid followed by enzymatic hydrolysis: the pretreatment conditions were expressed in a combined severity factor, CS, which ranged in the present study from -1.6 to 1.1. Considering the production of fermentable sugars and the release of inhibitory compounds, the optimal pretreatment conditions were 170 degrees C. 0% sulfuric acid and 60 min, corresponding to CS -0.4. Under these conditions, 21.4g glucose/L, 8.5 g xylose/L, and 0.5 g arabinose/L were produced, while 0.1 g HMF/L, 0.4 g furfural/L, 0.0 g levulinic acid/L, 0.0 g formic acid/L, and 2.1 g acetic acid/L were released. The ratio of Sigma sugars/Sigma inhibitors proved to be a good tool for evaluating the suitability of a hydrolysate for fermentation purposes. (C) 2011 Elsevier Ltd. All rights reserved.