A bench-scale system for monitoring butanol/acetone fermentations was designed to demonstrate the use of on-line gas chromatography (GC) and the applicability of an "advanced" fermentation equation. Ultrafiltration was used to obtain sterilely a cell-free sample from a controlled fermentor for on-line GC analysis of ethanol, acetone, acetate, butanol, acetoin, and butyrate. The fermentor effluent gas was also analyzed by on-line GC for nitrogen, carbon monoxide, and carbon dioxide. Analytical data were automatically transmitted to a VAX 11/75 computer, for data archiving and fermentation equation calculations. The system was demonstrated in studies of pH and CO effects on the metabolism of Clostridium acetobutylicum. The on-line analytical systems provided excellent real time data. Also, three potentially useful gateway sensors were developed from the fermentation equation: glucose concentration, NADH2 from FdH2 and excess ATP. These demonstrated the usefulness of the fermentation equation for determination of intracellular and extracellular parameters in transient operation.