A vacuum membrane distillation process using a microporous polytetrafluoro-ethylene (PTFE) membrane was developed to concentrate 2,3-butanediol from model 2,3-butanediol solutions and fermentation broths. A 0.22-mum PTFE membrane passed water vapor while retaining 2,3-butanediol. Water flux through the membrane was 4 kg/m2.h at 35-degrees-C and 12-14 kg/M2.h at 55-degrees-C. Medium components, especially yeast extract and fermentation broth, reduced flux as compared to model solutions. Using this process, butanediol was concentrated from 40 g/L to about 650 g/L in the model solution, and when integrated to a fed-batch reactor, 2,3-butanediol was concentrated to over 430 g/L.