A coupled fermentation-pervaporation process was operated continuously with on-line mass spectrometric gas analysis monitoring of product accumulation on both the upstream and the downstream sides of the membrane. Efficient coupling of the fermentation with pervaporation was attained when a steady state of ethanol production and removal was achieved with whey permeate containing high concentrations of lactose (> 8%) or by controlled lactose additions that also compensated for loss of liquid due to pervaporation. The combined system consists of a tubular membrane pervaporation module, directly connected to a stirred fermenter to form one circulation loop, kept at 38 degrees C, with both units operating under computer control. Mass spectrometric gas analysis of the CO2 gas evolved in the fermenter and the ethanol and water in the pervaporate on the downstream side of the membrane enabled us to follow the production of ethanol and its simultaneous removal. Membrane selectivity was calculated on-line and served to monitor the functioning of the membrane. Batch-wise-operated fermentation-pervaporation with Candida pseudotropicalis IF-513 yielded over 120 gl(-1) of concentrated ethanol solution using supplemented whey permeate containing 16% lactose. A steady state lasting for about 20 h was achieved with ethanol productivity of 20 g h(-1) (approx. 4 g l(-1) h(-1)). Membrane selectivity was over 8. Controlled feeding of concentrated lactose suspension in the whey permeate (350 gl(-1)) resulted in the continuous collection of 120-140 gl(-1) of ethanol pervaporate for 5 days, by which time salt accumulation hampered the fermentation. Medium refreshment restored the fermentative activity of the yeast cells and further extended the coupled process to over 9 days (200 h), when reversible membrane fouling occurred. The membrane module was exchanged and the combined process restarted.