To meet desired overall yields during ethanol production from lignocellulosic materials, it is important to use both hexoses and pentoses. Currently, genetically modified Saccharomyces cerevisiae strains are becoming available for xylose fermentation. However, simultaneous fermentation of xylose and glucose in genetically modified Saccharomyces cerevisiae requires a favorable ratio between these sugars. In simultaneous saccharification and fermentation (SSF) of spruce, which has been shown to be a viable process option for ethanol production, the ratio of xylose to glucose in the material is relatively low, which makes xylose fermentation challenging. However, by co-fermentation of the sugars in spruce, the theoretical ethanol yield can be increased by as much as 7-8%, which is significant in an industrial process. By performing enzymatic hydrolysis and model SSF experiments, studies of enzyme kinetics and consumption rates was carried out in the current work, in order to create a model for sugar release and sugar uptake. The model was then used to improve SSF experiments, and the most feasible SSF mode for co-fermentation of xylose and glucose, was established.