Pichia stipitis Y7124 was grown anaerobically on d-xylose in the presence of an initial ethanol concentration (E0) varying from 0 to 40 g/l. When E0 increased, the yield of xylitol increased linearly, reaching a value of 0.20 mol xylitol/mol xylose at E0=40 g/l. When a hydrogen acceptor (acetoin) was added to the cultures, the cylitol yield decreased with the contaminant stoichiometric reduction of acetoin to 2,3-butanediol. Furthermore, it was demonstrated that xylitol dehydrogenase and acetoin reductase activities from cell-free extracts of P. stipitis Y7124 were NAD+ and NADH2-linked, respectively. A hypothesis is put forward explaining that the xylitol yield is dependent on the ethanol concentration. It is suggested that ethanol may cause a disturbed NAD+/NADH2 balance during anaerobic xylose metabolism by P. stipitis. Metabolic mechanisms are proposed and their validity is discussed.