Pervaporation performance using silicalite membranes in the separation of an ethanol/water solution was affected by the addition of sugars, sugar alcohols or yeast cells. Although the membrane flux drastically decreased to about 30% of that for an aqueous ethanol solution with increasing glucose or lactose concentration, the selectivity towards ethanol was inversely enhanced by the addition of glucose from 23 to 137. The accumulated proteins and acidic by-products in the fermentation broth caused the decline in the performance.