In these studies, butanol (acetone butanol ethanol or ABE) was produced from concentrated lactose/whey permeate containing 211 g L-1 lactose. Fermentation of such a highly concentrated lactose solution was possible due to simultaneous product removal using a pervaporation membrane. In this system, a productivity of 0.43 g L-1 h(-1) was obtained which is 307 % of that achieved in a non-product removal batch reactor (0.14 g L-1 h(-1)) where approximately 60 g L-1 whey permeate lactose was fermented. The productivity obtained in this system is much higher than that achieved in other product removal systems (perstraction 0.21 g L-1 h(-1) and gas stripping 0.32 g L-1 h(-1)). This membrane was also used to concentrate butanol from approximately 2.50 g L-1 in the reactor to 755 g L-1. Using this membrane, ABE selectivities and fluxes of 24.4-44.3 and 0.57-4.05 g m(-2) h(-1) were obtained, respectively. Pervaporation restricts removal of water from the reaction mixture thus requiring significantly less energy for product recovery when compared to gas stripping.