BACKGROUND: Efficient conversion of glucose/xylose mixtures from lignocellulose is necessary for commercially viable ethanol production. Oxygen and carbon sources are of paramount importance for ethanol yield. The aim of this work was to evaluate different glucose/xylose mixtures for ethanol production using S. cerevisiae ITV-01 (wild type yeast) and P. stipitis NRRL Y-7124 and the effect of supplying oxygen in separate and co-culture processes. RESULTS: The complete conversion of a glucose/xylose mixture (75/30 g L(-1)) was obtained using P. stipitis NRRL Y-7124 under aerobic conditions (0.6 vvm), the highest yield production being Y(p/s) = 0.46 g g(-1), volumetric ethanol productivity Q(pmax) = 0.24 g L(-1) h(-1) and maximum ethanol concentration P(max) = 34.5 g L(-1). In the co-culture process and under aerobic conditions, incomplete conversion of glucose/xylose mixture was observed (20.4% residual xylose), with a maximum ethanol production of 30.3 g L(-1), ethanol yield of 0.4 g g(-1) and Q(pmax) = 1.26 g L(-1) h(-1). CONCLUSIONS: The oxygen present in the glucose/xylose mixture promotes complete sugar consumption by P. stipitis NRRL Y-7124 resulting in ethanol production. However, in co-culture with S. cerevisiae ITV-01 under aerobic conditions, incomplete fermentation occurs that could be caused by oxygen limitation and ethanol inhibition by P. stipitis NRRL Y-7124; nevertheless the volumetric ethanol productivity increases fivefold compared with separate culture. (C) 2011 Society of Chemical Industry