AbstractPervaporation using oleyl alcohol supported liquid membrane was successfully applied to diacetyl fermentation by immobilized lactic acid bacteria. Diacetyl productivity was about 10gm-3h-1, while productivity during batch fermentation was about 6gm-3h-1. Diacetyl yield from consumed glucose was about 0.04gg-1 which was 4 times as large as that of batch fermentation. The pervaporation functioned favorably on actual fermentation broth. The flux of the permeate and the diacetyl separation factor for the pervaporation were about 9gm-2h-1 and 36, respectively, and these values were maintained at almost constant levels during fermentation. Diacetyl concentration in the permeate was about 2kgm-3, which is sufficiently high for commercial use.