The chemical pretreatment with ozone of rye and wheat straws was carried out in a fixed bed reactor. The effect of ozone pretreatment time on lignin removal was determined. Glucose and xylose concentrations. in the hydrolysates were also measured after the subsequent enzymatic hydrolysis step. Acid insoluble lignin reacts with ozone within the first 90 min reaction. Insoluble lignin reduction was about 50%. The higher hydrolysis yields were obtained after 120 min ozonation: the glucose yield ranged from 40% to 50% for rye straw and from 34% to 39% for wheat straw, whereas xylose yields were about 30%, independently of the cereal straw. The glucose yields corresponding to the untreated raw straws were considerably lower, about 10%. Longer ozonation time sharply reduced the production of monosaccharides, probably due to the formation of side products. The kinetic model, with the reaction parameters estimated, predicted reasonably well the experimental data. (C) 2011 Elsevier Ltd. All rights reserved.