Substrate pretreatment is a cost-sensitive operation in the production of ethanol from lignocellulosic materials. The general purpose of this work is to optimize substrate pretreatment by selectively solubilizing the hemicellulose fraction to render a cellulose-enriched fraction for further fermentation or enzyme hydrolysis. Hemicellulose hydrolyzate, usually a waste stream, is proposed to be fermented by the pentose-utilizing yeast Pichia stipitis. Sugar beet pulp (SBP) was chosen as a substrate due to its low lignin content which makes substrate pretreatment simpler and less expensive. Hemicellulose utilization is very important in the case of SBP, whose hemicellulose content is as high as 50%. Conditions for SBP pretreatment by acid hydrolysis were optimized, SBP and acid (HCl and H2SO4) concentrations, agitation, temperature and particle size were analyzed as key variables. Experiments were planned according to a factorial design, the objective function to be optimized being the percentage ratio of hydrolyzed hemicellulose to hydrolyzed cellulose. SBP concentration was always more than 50 g/l to obtain a high sugar concentration in the hydrolyzate. The best conditions for SBP pretreatment were: 55 g/l of 32-50 mesh SBP, 1·1 g H2SO4/g SBP, 90 min at 80°C and 400 rpm. Under such conditions, 86·3% and 7·8% of cellulose and hemicellulose hydrolysis, respectively, were obtained.