A mathematical model of the membrane-extractive butanol fermentation process, which has been de-veloped as a method of in situ product recovery to solve the problems of butanol fermentation caused by the severe product inhibition, was proposed and the model was solved by a computer-assisted numerical method. The results in-dicate that the best way of improbing the butanol productivity is to use a feed with high glucose concentration. On the other hand, only marginal advantages are obtained by increasing the membrane area and/or the extractant flow rate beyond certain limits. It is also shown that a solvent with distribution coefficients as high as those of oleyl alcohol is ac-ceptable for this process.