A method of removing a fermentation inhibitor from an aqueous mixture of hydrolysis products from cellulosic biomass is disclosed. The method includes providing an aqueous mixture of hydrolysis products from cellulosic biomass, the hydrolysis products comprising at least one of pentose or hexose sugars and a fermentation inhibitor that inhibits a microorganism otherwise capable of fermenting pentose or hexose sugars; and at least partially extracting the fermentation inhibitor from the aqueous mixture with a first extractant by a first liquid-liquid extraction through a first porous membrane to provide a first extract and a raffmate, the first extractant having a water solubility of less than one percent by weight. In some embodiments, the aqueous mixture contains an insufficient amount of the microorganism to ferment the aqueous mixture. In some embodiments, the aqueous mixture is a fermentation broth. A membrane solvent extraction system for carrying out the method is also disclosed.