This paper investigates the production of ethanol from steam pretreated aspen. The optimal conditions for both the pretreatment and ethanol production were determined. In the former step the parameters investigated were temperature (180-220 degreesC) and residence time (2-6 min). The most effective combination was 214 degreesC and 6 min. The exploded substrates were detoxified in three ways, washing with water at 65 degreesC proving to be the most effective. The substrate was then converted into ethanol via simultaneous saccharification and fermentation. Influences of the reactor type (shaken flasks and stirred bioreactors) and process parameters (solid-to-liquid ratio, enzyme loading, and stirrer speed) have been investigated. The highest ethanol yield obtained from solid-to-liquid ratios of 0.20 g/g was 85% in shaken flasks and 79% in helical stirred bioreactors. In the former case, the ethanol concentration in the broth was 47 g/L. The fermentation unit returns a solid residue-with a calorific value of 5612 kcal/kg. The chemical oxygen demand due to compounds dissolved in the stillage is 28 800 mg of O-2/L. Test runs were carried out at bench and pilot scales.