Recombinant Saccharomyces cerevisiae, able to ferment the pentoses D-xylose and L-arabinose, was modified for improved fermentation rates and yields. Pentose fermentation is relevant when low cost raw materials such as plant hydrolysates are fermented to ethanol. The two most widespread pentose sugars in our biosphere are D-xylose and L-arabinose. S. cerevisiae is unable to ferment pentoses but has been engineered to do so; however rates and yields are low. The imbalance of redox cofactors (excess NADP and NADH are produced) is considered a major limiting factor. For the L-arabinose fermentation we identified an NADH-dependent L-xylulose reductase replacing the previously known NADPH-dependent enzyme. For D-xylose fermentation we introduced an NADP-dependent glyceraldehyde 3-phospate dehydrogenase to regenerate NADPH.