Baker’s yeast, Saccharomyces cerevisiae, is a GRAS microorganism and the most extensively used fermentation organism in industrial ethanol production due to its high ethanol productivity and tolerance to industrial fermentation conditions. However S. cerevisiae can not naturally ferment pentose sugars, which make up a significant fraction of the carbohydrates in agricultural residues and in hardwoods. Therefore metabolic engineering have been extensively explored to introduce pentose utilization pathways in S. cerevisiae. In the current study mayor enzymes of different xylose utilizing pathways were introduced in S. cerevisiae. The resulting strains were assessed with respect to enzyme kinetics and rate controlling metabolic reactions during growth in xylose medium.