Our approach however is to isomerize the xylose to xylulose using the exogenous enzyme Xylose Isomerase (XI) and then ferment the xylulose to ethanol by using native yeast strains in the same vessel.  Isomerization, however occurs optimally at a pH of 7-8 while subsequent fermentation step occurs at a pH of 4-5. We have successfully demonstrated that the maintenance of two vastly different pH microenvironments in a single vessel was possible using XI for isomerization co-immmobilized with an outer layer of the enzyme urease for pH control. As hydrogen ions diffuse from the fermentation broth into the pellet, they are neutralized by the ammonia produced in the hydrolysis of urea (added to fermentation media). Using our co-immobilized enzyme system with specific additives for driving the isomerization forward, we have obtained xylose conversions that are higher than those possible with the native XI pellets operating under optimal pH. The results of fermentation of pure xylose, mixed sugars and poplar hydrolysate from leading pretreatment technologies will be presented